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.

We study the in-plane anisotropy of the thermoelectric power and electrical resistivity on detwinned single crystals of isovalent substituted EuFe2(As1−xPx)2. Compared to the resistivity anisotropy the thermopower anisotropy is more pronounced and clearly visible already at temperatures much above the structural and magnetic phase transitions. Most remarkably, the thermopower anisotropy changes sign below the structural transition. This is associated with the interplay of two contributions due to anisotropic scattering and orbital polarization, which dominate at high-and low-temperatures, respectively.PACS numbers: 74.70.Xa;74.25.fg;74.40.Kb Electronic states with broken rotational symmetry driven by electronic correlations rather than the anisotropy of the underlying crystal lattice have recently attracted considerable attention [1][2][3][4][5]. The iron-pnictide superconductors provide a new way to explore the relation of superconductivity (SC) and electronic nematicity. The AFe 2 As 2 (A=Ba, Sr, Ca or Eu) ("122") materials crystallize in a tetragonal structure at high temperatures. Upon cooling through nearby structural (T s ) and magnetic (T N ) phase transitions, a low-temperature orthorhombic phase is stabilized where the Fe spins point along the (longer) a-axis with antiferromagnetic (AF) alignment [6]. Along the direction of the (shorter) b-axis, neighboring spins are coupled ferromagnetically. The orthorhombic lattice distortion results in the formation of twin domains at T < T s . A small uniaxial pressure along one of the orthorhombic in-plane directions is sufficient for detwinning [7].Evidence for a pronounced in-plane electronic anisotropy of 122 systems below T s has been found in the electrical resistivity [8], optical response to polarized light [9,10], quantum oscillations [11] and angular resolved photoemission spectroscopy (ARPES) [12]. The large energy separation of two orthogonal bands with predominant d xz and d yz character found in ARPES, sketched in the lower inset of Figure 1, indicates an orbital polarization at low temperatures [12].Remarkably, even for temperatures well above T s , uniaxial stress induces a pronounced resistivity anisotropy [8]. Using a piezo device, the resistivity anisotropy in the limit of zero strain has been detected [13]. Indeed, this "nematic susceptibility" diverges in the tetragonal state upon cooling from high T down to T s , even once the latter is suppressed towards T → 0 by doping. Importantly, electronic nematicity above T s has also been confirmed on micro crystals with presumed unbalanced twin-domain volumes by magnetic torque measurements [14].The origin of the resistivity anisotropy is controversially discussed. In one scenario, it is related to the orbital polarization, even at temperatures above T s [15,16]. An alternative scenario has been proposed in [17]. The columnar AF ground state of iron-pnictides has a discrete Ising-type symmetry, related to stripes of parallel spins along one of the in-plane axis. Consequently, both the spin rotation...

The in-plane thermal conductivity κ of electron-doped iron-arsenide superconductor BaFe1.9Ni0.1As2 (Tc = 20.3 K) single crystal was measured down to 70 mK. In zero field, the absence of a residual linear term κ0/T at T → 0 is strong evidence for nodeless superconducting gap. In magnetic field, κ0/T shows a slow field dependence up to H = 14.5 T (≈ 30% Hc 2 ). This is consistent with the superconducting gap structure demonstrated by angle-resolved photoemission spectroscopy experiments in BaFe1.85Co0.15As2 (Tc = 25.5 K), where isotropic superconducting gaps with similar size on hole and electron pockets were observed.

The electronic structure of a new charge-density-wave/ superconductor system, 1T-CuxTiSe2, has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case. With Cu doping, the charge density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states. Moreover, the strong scattering at high doping might be responsible for the suppression of superconductivity in that regime. [5,6,7], whereas it rarely exists in 1T structured compounds.Recently, the discovery of superconductivity in 1T-Cu x TiSe 2 has further enriched the physics of TMD's [8]. The undoped 1T-TiSe 2 is a CDW material, whose mechanism remains controversial after decades of research. For example, some considered the CDW a band-type Jahn-Teller effect, where the electronic energy is lowered through structural distortion [9,10]. Some considered it a realization of the excitonic CDW mechanism proposed by Kohn in the 1960's [11,12]; but different models were proposed to interpret the electronic structure, depending on whether system was argued to be a semimetal, or a semiconductor [13,14]. With Cu doping, it was found that the CDW transition temperature quickly drops, similar to other M x TiSe 2 's (M=Fe,Mn,Ta,V and Nb) [15,16,17,18]. Meanwhile, the superconducting phase emerges from x ∼ 0.04 and reaches the maximal transition temperature of 4.3K at x ∼ 0.08, then decreases to 2.8K at x ∼ 0.10. Quite remarkably, this phase diagram resembles those of the cuprate and heavy fermion superconductors [19], except here the competing order of superconductivity is the charge order, instead of the antiferromagnetic spin order. The presence of this ubiquitous phase diagram in 1T-Cu x TiSe 2 calls for a detailed study of its electronic structure. In particular, the information retrieved might help resolve the controversy on the CDW mechanism for 1T-TiSe 2 .We studied 1T-Cu x TiSe 2 with high resolution angle resolved photoemission spectroscopy (ARPES). A correlated semiconductor band structure of the undoped system is evidently illustrated, resolving a long-standing controversy. Cu doping is found to effectively enhance the density of states around the Fermi energy (E F ), which explains the enhancement of superconductivity. On the other hand, severe inelastic scattering was observed near the solubility limit, corresponding to the drop of superconducting transition temperature in that regime. With increased doping, chemical potential is raised, and signs of the weakening electron-hole coupling is discovered, which is responsible for the suppression of the CDW. Our results indicate that the seeming "competition" between CDW and superconductivity in the phase diagram is a coincidence caused by different effects of doping in this 1T compound, in contrast to the 2H-TMD case [3].1T-Cu x TiSe 2 single crystals were prepared by the vapor-transport technique, with doping x = 0, 0. 015, 0.025, 0.055, 0.065 and 0.11 (accurate within ...