Elastic constant measurements on iron pnictide Ba(Fe 0:9 Co 0:1 ) 2 As 2 with an optimal superconducting transition temperature of T SC ¼ 23 K have been performed by the ultrasonic pulse echo method. The shear elastic constant of C 66 associated with elastic strain " xy reveals considerable softening of 21% below 300 K down to T SC and becomes increasing in the superconducting phase below T SC , while other shear elastic constants of ðC 11 À C 12 Þ=2 and C 44 show no sign of softening below 80 K down to 4.2 K. The electric quadrupole O v 0 existing in the degenerate d y 0 z and d zx 0 bands participates in the superconductivity of the present iron pnictide system. KEYWORDS: layered iron pnictide, superconductor, Ba(Fe 0:9 Co 0:1 ) 2 As 2 , elastic softeningThe discovery of the superconductor LaFeAs(O 1Àx F x ) with a high transition temperature of T SC ¼ 26 K by Hosono's group triggered thousands of research works pursuing FeAs-based superconductors with various chemical composites and different structures.1) Five crystal structures with a common iron-based layer structure are now known.2)The high transition temperatures of up to 55 K and high critical magnetic fields beyond 50 T have already been realized.3,4) Among these FeAs-based crystals, the tetragonal compound of Ba(Fe 1Àx Co x ) 2 As 2 , in particular, has received much attention, because large single crystals are available for the investigation of physical properties depending on crystal anisotropy. 5)The end-material BaFe 2 As 2 exhibits a simultaneous transition of structural transition from a tetragonal TrCr 2 Si 2 -type structure of space group D orbitals by Co 2þ with 3d 7 orbitals in Ba(Fe 1Àx Co x ) 2 As 2 reduces both the structural transition temperature of T s and the antiferromagnetic one of T N .8) The superconductivity emerges in the systems above x ¼ 0:03 at the temperatures of T SC below the structural point T s and the antiferromagnetic one T N . The quantum critical point, x QCP , where both structural and antiferromagnetic phases die out, exists in the proximity of x ¼ 0:07. The optimum superconducting temperatures of T SC ¼ 23 K are achieved with x ¼ 0:10, which is a slightly excess of the quantum critical concentration x QCP . The superconductivity vanishes in the highly doped region of x ¼ 0:17.The Fermi surfaces of iron pnictides arise from Fe-3d orbitals and have three hole bands at a À-point and two electron bands at an X-point. These bands consist primarily of d y 0 z and d zx 0 with À 5 under tetragonal symmetry and d x 02 Ày 02 with À 3 .9) The increasing of the Fermi level upon electron doping due to the substitution of Fe with Co reduces the three sheets of hole bands around the À point in the endmaterial BaFe 2 As 2 to two sheets of hole bands in the present Ba(Fe 0:9 Co 0:1 ) 2 As 2 . We adopt the electric quadrupole associated with the degenerate d y 0 z and d zx 0 orbital states to describe elastic properties of the compound Ba(Fe 1Àx Co x ) 2 -As 2 . It is noted that the coordinates x 0 and y 0 of the quadrupole are ori...

Unconventional features of relativistic Dirac/Weyl quasi-particles in topological materials are most evidently manifested in the 2D quantum Hall effect (QHE), whose variety is further enriched by their spin and/or valley polarization. Although its extension to three dimensions has been long-sought and inspired theoretical proposals, material candidates have been lacking. Here we have discovered valley-contrasting spin-polarized Dirac fermions in a multilayer form in bulk antiferromagnet BaMnSb 2 , where the out-of-plane Zeeman-type spin splitting is induced by the in-plane inversion symmetry breaking and spin-orbit coupling (SOC) in the distorted Sb square net. Furthermore, we have observed well-defined quantized Hall plateaus together with vanishing interlayer conductivity at low temperatures as a hallmark of the half-integer QHE in a bulk form.The Hall conductance of each layer is found to be nearly quantized to 2(N+1/2)e 2 /h with N being the Landau index, which is consistent with two spin-polarized Dirac valleys protected by the strong spin-valley coupling. arXiv:2001.08683v1 [cond-mat.str-el] 23 Jan 2020 Researches of topological materials have currently been one of the central topics of the condensed matter physics. Their topologically non-trivial electronic structure leads to the relativistic quasiparticles, Dirac/Weyl fermions, whose most prominent feature is seen in QHE in 2D systems[1], such as the relativistic QHE in graphene[2, 3] and topological insulator films[4]. The half-integer quantization of the Hall plateaus and the zero-energy Landau level forming at the charge neutral Dirac point were experimentally clarified, which are associated with the Berry phase of Dirac fermions and hence have no analog in conventional 2D systems. More recently, the variety of QHE in topological materials has been further expanded by utilizing the spin and/or valley polarization in the system[5-19K21851, JP19H05173) and the Asahi Glass Foundation. The synchrotron radiation experiments were performed at the BL25SU of SPring-8 with the approval of the Japan Synchrotron

The pseudogap phenomenon in the cuprates is arguably the most mysterious puzzle in the field of high-temperature superconductivity. The tetragonal cuprate HgBa 2 CuO 4+ δ , with only one CuO 2 layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the magnetic susceptibility anisotropy within the CuO 2 plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy sets in below the pseudogap temperature T * , which provides thermodynamic evidence for a nematic phase transition with broken four-fold symmetry. Surprisingly, the nematic director orients along the diagonal direction of the CuO 2 square lattice, in sharp contrast to the bond nematicity along the Cu-O-Cu direction. Another remarkable feature is that the enhancement of the diagonal nematicity with decreasing temperature is suppressed around the temperature at which short-range charge-density-wave formation occurs. Our result suggests a competing relationship between diagonal nematic and charge-density-wave order in HgBa 2 CuO 4+ δ .

Unconventional surface states protected by non-trivial bulk orders are sources of various exotic quantum transport in topological materials. One prominent example is the unique magnetic orbit, so-called Weyl orbit, in topological semimetals where two spatially separated surface Fermi-arcs are interconnected across the bulk. The recent observation of quantum Hall states in Dirac semimetal Cd 3 As 2 bulks have drawn attention to the novel quantization phenomena possibly evolving from the Weyl orbit. Here we report surface quantum oscillation and its evolution into quantum Hall states in Cd 3 As 2 thin film samples, where bulk dimensionality, Fermi energy, and band topology are systematically controlled. We reveal essential involvement of bulk states in the quantized surface transport and the resultant quantum Hall degeneracy depending on the bulk occupation. Our demonstration of surface transport controlled in film samples also paves a way for engineering Fermi-arc-mediated transport in topological semimetals.

A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A long-standing open problem, which is a key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS2 is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted and electron irradiated crystals of 1T-TaS2, where the substitution of S by Se induces weak disorder and electron irradiation induces strong quenched disorder. In pure 1T-TaS2, the linear temperature term of the heat capacity γT and the finite residual linear term of the thermal conductivity in the zero-temperature limit κ0/T ≡ κ/T (T → 0) are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances κ0/T , it strongly suppresses γ. These unusual contrasting responses to magnetic field imply the coexistence of two types of gapless excitations with itinerant and localized characters. Introduction of additional weak random exchange disorder in 1T-Ta(S1−xSex)2 leads to vanishing of κ0/T , indicating that the itinerant gapless excitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magnetic contribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumes the presence of localized orphan spins forming random singlets. These results appear to capture an essential feature of the QSL state of 1T-TaS2; localized orphan spins induced by disorder form random valence bonds and are surrounded by a QSL phase with spinon Fermi surface. Electron irradiation in pure 1T-TaS2 largely enhances γ and changes the scaling function dramatically, suggesting a possible new state of spin liquid.