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Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

We present single-crystal neutron scattering measurements of the spin-1/2 equilateral triangular-lattice antiferromagnet Ba3CoSb2O9. Besides confirming that the Co 2+ magnetic moments lie in the ab plane for zero magnetic field and then determining all the exchange parameters of the minimal quasi-2D spin Hamiltonian, we provide conclusive experimental evidence of magnon decay through observation of intrinsic line-broadening. Through detailed comparisons with the linear and nonlinear spin-wave theories, we also point out that the large-S approximation, which is conventionally employed to predict magnon decay in noncollinear magnets, is inadequate to explain our experimental observation. Thus, our results call for a new theoretical framework for describing excitation spectra in low-dimensional frustrated magnets under strong quantum effects. The equilateral triangular-lattice quantum antiferromagnet Ba 3 CoSb 2 O 9 was synthesized recently [24][25][26][27][28][29]. The Co 2+ ion has a Kramers doublet ground-state due to the spin-orbit coupling, and this doublet can be described as an effective spin-1/2 moment. In addition, the high symmetry of the hexagonal crystal structure, P6 3 / mmc [24-28], forbids DM interaction for pairs up to third nearest-neighbor (NN) in the same abplane and between any pair of spins along the c-axis [25].Powder neutron diffraction measurements presented the noncollinear 120• structure with the magnetic wavevector Q = (1/3, 1/3, 1) [24]. The Néel temperature was found to be ≈ 3.8 K and a rich temperature-magnetic field phase diagram was reported up to 32 T [25][26][27][28]. Electronic spin resonance (ESR) [27] and nuclear magnetic resonance (NMR) [28] measurements suggested a spin model with small easy-plane exchange anisotropy and an exchange interaction along the caxis much weaker than the NN intralayer exchange. This observation is consistent with the alternation of magnetic (Co 2+ ) and nonmagnetic (Sb 2 O 9 bioctahedra) layers along the cdirection. While more precise determination of the model parameters requires inelastic neutron scattering (INS) measurements, such detailed information is indeed physically relevant.

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe2As2 (T_{N}=200-220 K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,Planck's omega) consists of a Bragg peak at the elastic position (Planck's omega=0 meV), a spin gap (Delta< or =6.5 meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T_{N}, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe2As2 is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

Much of modern condensed matter physics is understood in terms of elementary excitations, or quasiparticles-fundamental quanta of energy and momentum 1,2 . Various strongly interacting atomic systems are successfully treated as a collection of quasiparticles with weak or no interactions. However, there are interesting limitations to this description: in some systems the very existence of quasiparticles cannot be taken for granted. Like unstable elementary particles, quasiparticles cannot survive beyond a threshold where certain decay channels become allowed by conservation laws; their spectrum terminates at this threshold. Such quasiparticle breakdown was first predicted for an exotic state of matter-super-fluid 4 He at temperatures close to absolute zero, a quantum Bose liquid where zero-point atomic motion precludes crystallization 1-4 . Here we show, using neutron scattering, that quasiparticle breakdown can also occur in a quantum magnet and, by implication, in other systems with Bose quasiparticles. We have measured spin excitations in a two-dimensional quantum magnet, piperazinium hexachlorodicuprate (PHCC) 5 , in which spin-1/2 copper ions form a non-magnetic quantum spin liquid, and find remarkable similarities with excitations in superfluid 4 He. We observe a threshold momentum beyond which the quasiparticle peak merges with the two-quasiparticle continuum. It then acquires a finite energy width and becomes indistinguishable from a leading-edge singularity, so that excited states are no longer quasiparticles but occupy a wide band of energy. Our findings have important ramifications for understanding excitations with gapped spectra in many condensed matter systems, ranging from band insulators to high-transition-temperature superconductors 6 . Although of all the elements only liquid helium fails to crystallize at temperature T ¼ 0, quantum liquids are quite common in condensed matter. Metals host electron Fermi liquids, and superconductors contain Bose liquids of Cooper pairs. Trapped ultracold atoms can also form quantum liquids, and some remarkable new examples were recently identified in magnetic crystals 5,7-10 . The organometallic material PHCC is an excellent physical realization of a quantum spin liquid (QSL) in a two-dimensional (2D) Heisenberg antiferromagnet (HAFM). Its Cu 2þ spins are coupled through a complex network of orbital overlaps, and form an array of slightly skewed anisotropic spin-1/2 ladders 10 in the crystalline a-c plane with highly frustrated super-exchange interactions 5 . The spin excitations in PHCC have a spectral gap D s < 1 meVand nearly isotropic 2D dispersion in the (h0l) plane with a bandwidth slightly larger than D s . In the absence of a magnetic field, only the short-range dynamic spin correlations typical of a liquid exist: the spin gap precludes longrange magnetic order down to T ¼ 0. Here we explore magnetic excitations in PHCC via inelastic neutron scattering and compare the results with similar measurements in the quantum fluid 4 He, emphasizing the effects ...

We have determined the full magnetic dispersion relations of multiferroic BiFeO3. In particular, two excitation gaps originating from magnetic anisotropies have been clearly observed. The direct observation of the gaps enables us to accurately determine the Dzyaloshinskii-Moriya (DM) interaction and the single ion anisotropy. The DM interaction supports a sizable magnetoelectric coupling in this compound.

The spin-1=2 triangular lattice antiferromagnet YbMgGaO 4 has attracted attention recently as a quantum spin-liquid candidate with the possible presence of off-diagonal anisotropic exchange interactions induced by spin-orbit coupling. Whether a quantum spin liquid is stabilized or not depends on the interplay of various exchange interactions with chemical disorder that is inherent to the layered structure of the compound. We combine time-domain terahertz spectroscopy and inelastic neutron scattering measurements in the field-polarized state of YbMgGaO 4 to obtain better insight of its exchange interactions. Terahertz spectroscopy in this fashion functions as a high-field electron spin resonance and probes the spin-wave excitations at the Brillouin zone center, ideally complementing neutron scattering. A global spin-wave fit to all our spectroscopic data at fields over 4 T, informed by the analysis of the terahertz spectroscopy linewidths, yields constraints on the disorder-averaged g factors and exchange interactions. Our results paint YbMgGaO 4 as an easy-plane XXZ antiferromagnet with the combined and necessary presence of subleading next-nearest neighbor and weak anisotropic off-diagonal nearest-neighbor interactions. Moreover, the obtained g factors are substantially different from previous reports. This work establishes the hierarchy of exchange interactions in YbMgGaO 4 from high-field data alone and thus strongly constrains possible mechanisms responsible for the observed spin-liquid phenomenology.

Low-temperature states of polycrystalline samples of a frustrated pyrochlore oxide Tb2+xTi2−xO7+y have been investigated by specific heat, magnetic susceptibility, and neutron scattering experiments. We have found that this system can be tuned by a minute change of x from a spin-liquid state (x < xc) to a partly ordered state with a small antiferromagnetic ordering of the order of 0.1µB. Specific heat shows a sharp peak at a phase transition at Tc = 0.5 K for x = 0.005. Magnetic excitation spectra for this sample change from a quasielastic to a gapped type through Tc. The possibility of a Jahn-Teller transition is discussed.PACS numbers: 75.10. Kt, 75.40.Cx, 75.70.Tj, 78.70.Nx Magnetic systems with geometric frustration, a prototype of which is antiferromagnetically coupled Ising spins on a triangle, have been intensively studied experimentally and theoretically for decades 1 . Spin systems on networks of triangles or tetrahedra, such as triangular 2 , kagomé 3 , and pyrochlore 4 lattices, play major roles in these studies. Subjects that have fascinated many investigators in recent years are classical and quantum spin-liquid states 5-8 , where conventional long-range order (LRO) is suppressed to very low temperatures. Quantum spin-liquids 6,7 in particular have been challenging both theoretically and experimentally since the proposal of the resonating valence-bond state 9 . The spin ice materials R 2 Ti 2 O 7 (R = Dy, Ho) are the well-known classical examples 5 , while other experimental candidates found recently have been studied 10-14 .Among frustrated pyrochlore oxides 4 , Tb 2 Ti 2 O 7 has attracted much attention because it does not show any conventional LRO down to 50 mK and remains in a dynamic spin-liquid state [15][16][17] . Theoretical considerations of the crystal-field (CF) states of Tb 3+ and exchange and dipolar interactions of the system [18][19][20] showed that it should undergo a transition into a magnetic LRO state at about 1.8 K within a random phase approximation 20 . The puzzling origin of the spin-liquid state of Tb 2 Ti 2 O 7 is a subject of hot debate 4,21-28 . An interesting scenario for the spin-liquid state is the theoretical proposal of a quantum spin-ice state 22 . More recently, another scenario of a two-singlet spin-liquid state was proposed to explain why inelastic neutron spectra in a low energy range are observed despite the fact that Tb 3+ is a nonKramers ion 23,24 .Several experimental puzzles of Tb 2 Ti 2 O 7 originate from the difficulty of controlling the quality of single crystalline samples, resulting in strongly sampledependent specific-heat anomalies at temperatures below 2 K 18,26,[29][30][31][32][33] . In contrast, experimental results on polycrystalline samples are more consistent 15,16,26 . Among the experimental results reported to date, an important clue to solve the puzzles of Tb 2 Ti 2 O 7 seems to be a change of state at about 0.4 K suggested by specific heat 26 , inelastic neutron scattering 26 , and neutron spin echo 16 measurements on polycrystalline samp...