Understanding and controlling microscopic heat transfer mechanisms in solids is critical to material design in numerous technological applications. Yet, the current understanding of thermal transport in semiconductors and insulators is limited by the difficulty in directly measuring individual phonon lifetimes and mean free paths, and studying their dependence on the microscopic state of the material. Here we report our measurements of microscopic phonon scattering rates in AgBiSe2, which exhibits a controllable, reversible change directly linked to microstructure evolution near a reversible structural phase transition, that directly impacts the thermal conductivity. We demonstrate a steplike doubling of phonon scattering rates resultant from the cation disordering at the structural transition. To rationalize the neutron scattering data, we leverage a stepwise approach to account for alterations to the thermal conductivity that are imparted by distinct scattering mechanisms. These results highlight the potential of tunable microstructures housed in a stable crystal matrix to provide a practical route to tailor phonon scattering to optimize thermal transport properties.

Nematicity is ubiquitous in electronic phases of high-T_c superconductors, particularly in the Fe-based systems. While several experiments have probed nematic fluctuations, they have been restricted to uniform or momentum averaged fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by utilizing the anomalous softening of acoustic phonon modes in optimally doped Ba(Fe_0.94Co_0.06)_2As_2. We determine the nematic correlation length and find that it sharply changes its T-dependence at T_c, revealing a strong connection between nematicity and superconductivity.

J. A. Sears, M. Songvilay, K. W. Plumb, J. P. Clancy, Y. Qiu, 3 Y. Zhao, 3 D. Parshall, and Young-June Kim ∗ Department of Physics, University of Toronto, 60 St. George St., Toronto, Ontario, M5S 1A7, Canada NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899, USA Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA (Dated: July 20, 2018)

BaSnO3 has attracted attention as a transparent conducting oxide with high room temperature carrier mobility. We report a series of measurements that were carried out to assess the structure of BaSnO3 over a variety of length scales. Measurements included single crystal neutron and x-ray diffraction, Rietveld and pair distribution analysis of neutron powder diffraction, Raman scattering, and high-pressure x-ray diffraction. Results from the various diffraction probes indicate that both the long-range and local structures are consistent with the cubic symmetry. The diffraction data under pressure was consistent with a robustly cubic phase up to 48.9 GPa, which is supported by density functional calculations. Additionally, transverse phonon velocities were determined from measured dispersion of the transverse acoustic phonon branches, the results of which are in good agreement with previous theoretical estimates and ultrasound measurements.

Nematic order is ubiquitous in liquid crystals and is characterized by a rotational symmetry breaking in an otherwise uniform liquid. Recently a similar phenomenon has been observed in some electronic phases of quantum materials related to high temperature superconductivity, particularly in the Fe-based superconductors. While several experiments have probed nematic fluctuations, they have been primarily restricted to the uniform nematic susceptibility, i.e. q = 0 fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by measuring transverse acoustic phonon modes with wavelengths of up to 25 unit cells in the prototypical Fe-based compound Ba(Fe_1-xCo_x)_2As_2. While the slope of the phonon dispersion gives information about the uniform nematic susceptibility, deviations from this linear behavior at finite but small wave-vectors are attributed to finite-momentum nematic fluctuations. Surprisingly, these non-zero q fluctuations lead to a softening of the phonon mode below the superconducting transition temperature, in contrast to the behavior of the phonon velocity at q = 0, which increases below T_c. Our work not only establishes a sound method to probe long wavelength nematic fluctuations, but also sheds light on the unique interplay between nematicity and superconductivity in Fe-based compounds.

Nematic order is ubiquitous in liquid crystals and is characterized by a rotational symmetry breaking in an otherwise uniform liquid. Recently a similar phenomenon has been observed in some electronic phases of quantum materials related to high temperature superconductivity, particularly in the Fe-based superconductors. While several experiments have probed nematic fluctuations, they have been primarily restricted to the uniform nematic susceptibility, i.e. q = 0 fluctuations. Here, we investigate the behavior of finite-momentum nematic fluctuations by measuring transverse acoustic phonon modes with wavelengths of up to 25 unit cells in the prototypical Fe-based compound Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. While the slope of the phonon dispersion gives information about the uniform nematic susceptibility, deviations from this linear behavior at finite but small wave-vectors are attributed to finite-momentum nematic fluctuations. Surprisingly, these non-zero q fluctuations lead to a softening of the phonon mode below the superconducting transition temperature, in contrast to the behavior of the phonon velocity at q = 0, which increases below $T_c$. Our work not only establishes a sound method to probe long wavelength nematic fluctuations, but also sheds light on the unique interplay between nematicity and superconductivity in Fe-based compounds.

Nematic order is ubiquitous in liquid crystals and is characterized by a preferred direction in an otherwise isotropic liquid. Recently a similar symmetry breaking has been observed in some electronic phases in quantum materials related to high temperature superconductors. We investigated the prototypical Fe-based high-temperature superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ where the interplay of electronic nematic order and superconductivity is well established. Previous measurements found a strong suppression of nematic fluctuations at zero momentum in the superconducting state, but what happens at nonzero momentum is unknown. We found that fluctuations at small momenta, which correspond to wavelengths of as much as 25 unit cells, do not compete with superconductivity. Instead they continue to grow below superconducting $T_c$. This result contrasts the conventional picture where the behavior at the ordering wavevector ($mathbf{q}=0$) and nearby (small) wavevectors should be similar. Our results imply the existence of a length scale larger than 25 unit cells that is important for understanding of the interplay between nematicity and superconductivity.

Superlattice diffraction peaks have been recently observed in underdoped YBa2Cu3O6.6. They have been interpreted to originate from a charge density wave (CDW) formation. It is believed that strong phonon anomalies previously observed near the same wavevectors are related to the CDW. Competition with superconductivity is a salient feature of the CDW peaks. We investigated if the same is true of anomalous bond-stretching phonons around 60 meV by measuring the spectrum of this phonon at 10 K with and without an applied magnetic field of 10 T. Applying the field had no effect on the phonon within the experimental uncertainty.

The evolution of the electronic properties of electron-doped (Sr1-xLax)2IrO4 is experimentally explored as the doping limit of La is approached. As electrons are introduced, the electronic ground state transitions from a spin-orbit Mott phase into an electronically phase separated state, where long-range magnetic order vanishes beyond x = 0.02 and charge transport remains percolative up to the limit of La substitution (x 0.06). In particular, the electronic ground state remains inhomogeneous even beyond the collapse of the parent state's long-range antiferromagnetic order, while persistent short-range magnetism survives up to the highest La-substitution levels. Furthermore, as electrons are doped into Sr2IrO4, we observe the appearance of a low temperature magnetic glass-like state intermediate to the complete suppression of antiferromagnetic order. Universalities and differences in the electron-doped phase diagrams of single layer and bilayer Ruddlesden-Popper strontium iridates are discussed.

We present time-of-flight inelastic neutron scattering measurements of La1.965Ba0.035CuO4 (LBCO), a lightly doped member of the high temperature superconducting La-based cuprate family. By using time-of-flight neutron instrumentation coupled with single crystal sample rotation we obtain a four-dimensional data set (three Q and one energy) that is both comprehensive and spans a large region of reciprocal space. Our measurements identify rich structure in the energy dependence of the highly dispersive spin excitations, which are centered at equivalent (1/2, 1/2, L) wave vectors. These structures correlate strongly with several crossings of the spin excitations with the lightly dispersive phonons found in this system. These effects are significant and account for on the order of 25% of the total inelastic scattering for energies between approximate to 5 and 40 meV at low |Q|. Interestingly, this scattering also presents little or no L dependence. As the phonons and dispersive spin excitations centered at equivalent (1/2, 1/2, L) wave vectors are common to all members of La-based 214 copper oxides, we conclude such strong quasi-two-dimensional scattering enhancements are likely to occur in all such 214 families of materials, including those concentrations corresponding to superconducting ground states. Such a phenomenon appears to be a fundamental characteristic of these materials and is potentially related to superconducting pairing.

We report magnetic and thermodynamic properties of single crystal alpha-RuCl3, in which the Ru3+ (4d(5)) ion is in its low spin state and forms a honeycomb lattice. Two features are observed in both magnetic susceptibility and specific heat data; a sharp peak at 7 K and a broad hump near 10-15 K. In addition, we observe a metamagnetic transition between 5 and 10 T. Our neutron diffraction study of single crystal samples confirms that the low temperature peak in the specific heat is associated with a magnetic order with unit cell doubling along the honeycomb (100) direction, which is consistent with zigzag order, one of the types of magnetic order predicted within the framework of the Kitaev-Heisenberg model.

Parent compounds of Fe-based superconductors undergo a structural phase transition from a tetragonal to an orthorhombic structure. We investigated the temperature dependence of the frequencies of TA phonons that extrapolate to the shear vibrational mode at the zone center, which corresponds to the orthorhombic deformation of the crystal structure at low temperatures in BaFe2As2 and SrFe2As2. We found that acoustic phonons at small wave vectors soften gradually towards the transition from high temperatures, tracking the increase of the size of slowly fluctuating magnetic domains. On cooling below the transition to base temperature the phonons harden, following the square of the magnetic moment (which we find is proportional to the anisotropy gap). Our results provide evidence for close correlation between magnetic and phonon properties in Fe-based superconductors.

The ferropnictide superconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes that are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23 meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in 40 Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to 0.1 meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons.