We report the quantum transport properties of Cd₃As₂ single crystals in a magnetic field. A large linear quantum magnetoresistance is observed near room temperature. With decreasing temperature, the Shubnikov-de Haas oscillations appear in both the longitudinal resistance R(xx) and the transverse Hall resistance R(xy). From the strong oscillatory component ΔR(xx), a linear dependence of the Landau index n on 1/B is obtained, and it gives an n-axis intercept between 1/2 and 5/8. This clearly reveals a nontrivial π Berry's phase, which is a distinguished feature of Dirac fermions. Our quantum transport results provide bulk evidence for the existence of a three-dimensional Dirac semimetal phase in Cd₃As₂.

The in-plane resistivity rho and thermal conductivity kappa of the FeAs-based superconductor KFe2As2 single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior rho(T) approximately T{1.5} at H{c{2}}=5 T, and the development of a Fermi liquid state with rho(T) approximately T{2} when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field H{c{2}}. In zero field, there is a large residual linear term kappa{0}/T, and the field dependence of kappa_{0}/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

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Na4Ir3O8 is a candidate material for a 3-dimensional quantum spin-liquid on the hyperkagome lattice. We present thermodynamic measurements of heat capacity C and thermal conductivity κ on high quality polycrystalline samples of Na4Ir3O8 down to T = 500 mK and 75 mK, respectively. Absence of long-range magnetic order down to T = 75 mK strongly supports claims of a spin-liquid ground state. The constant magnetic susceptibility χ below T ≈ 25 K and the presence of a small but finite linear-T term in C(T ) suggest the presence of gapless spin excitations. Additionally, the magnetic Grüneisen ratio shows a divergence as T → 0 K and a scaling behavior which clearly demonstrates that Na4Ir3O8 is situated close to a zero-field QCP.In geometrically frustrated materials long range order is prohibited and a liquid-like ground state of spins, the spinliquid (SL) state, can occur. Since Anderson's proposal of a valence bond state for a triangular lattice [1], search for such quantum spin liquids (QSL) has been pursued intensely. In the past decade several new materials have been proposed as SL candidates (see recent reviews 2-4). A small spin and quasi-low-dimensionality are considered ingredients which can lead to a SL state. Indeed, most of the proposed QSL candidates are materials with S = 1/2 moments sitting on quasi-low-dimensional structures. These include the 2-dimensional (2D) triangular lattice organic compounds κ- on a frustrated hyperkagome lattice has been proposed as a candidate QSL [11]. Using thermodynamic measurements at T ≥ 2 K it was shown that Na 4 Ir 3 O 8 does not order inspite of strong antiferromagnetic interactions (θ = −650 K). The magnetic specific heat showed a bump around 30 K, a temperature much smaller than θ, and displays a low temperature power-law dependence with exponent close to 2 [11] similar to several of the 2D materials listed above [8,10]. Subsequently using classical and semi-classical spin-models of Heisenberg spins on a hyperkagome lattice the ground state was found to be highly degenerate and either a classical spin nematic order with long range dipolar spin correlations is chosen at T ≈ 1 K by an order-by-disorder mechanism [12] or a 120 o coplanar magnetically ordered state [13] were predicted. This coplanar magnetic order was found to give way, through a quntum phase transition, to a gapped topological Z 2 'bosonic' spin liquid phase (characterized by the absence of long-range dipolar order) when quantum fluctuations are turned on [13]. The elementary excitations of this gapped spin liquid state were predicted to be chargeless S = 1/2 spinons [13]. The prediction of a gapped spin liquid is however at odds with experiments which gave a Sommerfeld's coefficient γ ≈ 2 mJ/Ir mol K 2 suggesting gapless spin excitations unless the gap was vanishingly small or had nodes. In a completely different line of approach a Fermionic spin liquid model was developed [14,15]. This led naturally to gapless spin liquids as stable phases. These spin liquids had a Fermi surface of chargeless spinons ...

The in-plane thermal conductivity of the iron selenide superconductor FeSe x ͑T c = 8.8 K͒ was measured down to 120 mK and up to 14.5 T ͑Ӎ3 / 4H c 2 ͒. In zero field, the residual linear term 0 / T at T → 0 is only about 16 W K −2 cm −1 , less than 4% of its normal-state value. Such a small 0 / T does not support the existence of nodes in the superconducting gap. More importantly, the field dependence of 0 / T in FeSe x is very similar to that in NbSe 2 , a typical multigap s-wave superconductor. We consider our data as strong evidence for multigap nodeless ͑at least in ab plane͒ superconductivity in FeSe x . This kind of superconducting gap structure may be generic for all Fe-based superconductors.