A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states [1][2][3][4] , most of which are characterized by exotic spin excitations with fractional quantum numbers (termed 'spinon'). Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in a triangular antiferromagnet YbMgGaO 4 . The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to a QSL state with a spinon Fermi surface in YbMgGaO 4 that has a perfect spin-1/2 triangular lattice as in the original proposal 4 of quantum spin liquids.In 1973, Anderson proposed the pioneering idea of the quantum spin liquid (QSL) in the study of the triangular lattice Heisenberg antiferromagnet 4 . This idea was revived after the discovery in 1986 of high-temperature superconductivity 5 . A QSL, as currently understood, does not fit into Landau's conventional paradigm of symmetry breaking phases 1,2,6,7 , and is arXiv:1607.02615v2 [cond-mat.str-el] 31 Jul 2017 2 instead an exotic state of matter characterized by spinon excitations and emergent gauge structures [1][2][3]6 . The search for QSLs in models and materials [8][9][10][11][12] has been partly facilitated by the Oshikawa-Hastings-Lieb-Schultz-Mattis (OHLSM) theorem that may hint at the possibility of QSLs in Mott insulators with odd electron fillings and a global U(1) spin rotational symmetry [13][14][15] .Indeed, a continuum of spin excitations has been observed in a kagome-lattice material ZnCu 3 (OD) 6 Cl 2 (refs 12,16). However, the requirement of the U(1) spin rotational symmetry, prevents the application of OHLSM theorem in strong spin-orbit-coupled (SOC) Mott insulators in which the spin rotational symmetry is completely absent. A recent theory addressed this limitation of the OHLSM theorem, arguing that, as long as time-reversal symmetry is preserved, the ground state of an SOC Mott insulator with odd electron fillings must be exotic 17 .The newly discovered triangular antiferromagnet YbMgGaO 4 (refs 18,19) displays no indication of magnetic ordering or symmetry breaking at temperatures as low as 30 mK despite approximately 4 K for the spin interaction energy scale. Because of the strong SOC of the Yb electrons, YbMgGaO 4 was the first QSL to be proposed beyond the OHLSM theorem 19 . The thirteen 4 f electrons of the Yb 3+ ion form the spin-orbit-entangled Kramers doublets that are split by the D 3d crystal electric fields [20][21][22] . At temperatures co...
Thermoelectric technology allows conversion between heat and electricity. Many good thermoelectric materials contain rare or toxic elements, so developing low-cost and high-performance thermoelectric materials is warranted. Here, we report the temperature-dependent interplay of three separate electronic bands in hole-doped tin sulfide (SnS) crystals. This behavior leads to synergistic optimization between effective mass (m*) and carrier mobility (μ) and can be boosted through introducing selenium (Se). This enhanced the power factor from ~30 to ~53 microwatts per centimeter per square kelvin (μW cm−1 K−2 at 300 K), while lowering the thermal conductivity after Se alloying. As a result, we obtained a maximum figure of merit ZT (ZTmax) of ~1.6 at 873 K and an average ZT (ZTave) of ~1.25 at 300 to 873 K in SnS0.91Se0.09 crystals. Our strategy for band manipulation offers a different route for optimizing thermoelectric performance. The high-performance SnS crystals represent an important step toward low-cost, Earth-abundant, and environmentally friendly thermoelectrics.
Magnetic excitations of the filled skutterudite PrRu4P12 exhibiting a metal-insulator (M-I) transition at TM−I = 63 K were studied by inelastic neutron scattering experiment. The spectra at temperatures much lower than TM−I are described as well-defined crystal-field excitations. With approaching TM−I, the excitation peaks broaden and shift considerably together with the temperature variation of the carrier number and the atomic displacement in the transition. The 4f-electron state evolve from the well localized state in the insulator phase to the strongly hybridized itinerant state by p-f mixing near TM−I. The hybridization is responsible for the M-I transition of PrRu4P12.
The origin of nematic order remains one of the major debates in iron-based superconductors. In theories based on spin nematicity, one major prediction is that the spin-spin correlation length at (0,π) should decrease with decreasing temperature below the structural transition temperature T_{s}. Here, we report inelastic neutron scattering studies on the low-energy spin fluctuations in BaFe_{1.935}Ni_{0.065}As_{2} under uniaxial pressure. Both intensity and spin-spin correlation start to show anisotropic behavior at high temperature, while the reduction of the spin-spin correlation length at (0,π) happens just below T_{s}, suggesting the strong effect of nematic order on low-energy spin fluctuations. Our results favor the idea that treats the spin degree of freedom as the driving force of the electronic nematic order.
Materials with intrinsically low thermal conductivity are of fundamental interests. Here we report a new sort of simple one-dimensional (1D) crystal structured bismuth selenohalides (BiSeX, X = Br, I) with extremely low thermal conductivity of~0.27 W m −1 K −1 at 573 K. The mechanism of the extremely low thermal conductivity in 1D BiSeX is elucidated systematically using the first-principles calculations, neutron powder-diffraction measurements and temperature tunable aberration-corrected scanning transmission electron microscopy (STEM). Results reveal that the 1D structure of BiSeX possesses unique soft bonding character, low phonon velocity, strong anharmonicity of both acoustic and optical phonon modes, and large off-center displacement of Bi and halogen atoms. Cooperatively, all these features contribute to the minimal phonon transport. These findings provide a novel selection rule to search low thermal conductivity materials with potential applications in thermoelectrics and thermal barrier coatings.
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