In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE) [1,2]. Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co 2 MnGa, reaching S yx ∼ −6 µV/K at room T , one order of magnitude larger than the maximum value reported for a magnetic conductor [3]. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient α yx shows a crossover between T -linear and −T log(T ) behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions [4-6] is responsible for the observed crossover properties and an enhanced α yx . The

We study a possible superconductivity in quasiperiodic systems, by portraying the issue within the attractive Hubbard model on a Penrose lattice. Applying a real-space dynamical mean-field theory to the model consisting of 4181 sites, we find a superconducting phase at low temperatures. Reflecting the nonperiodicity of the Penrose lattice, the superconducting state exhibits an inhomogeneity. According to the type of the inhomogeneity, the superconducting phase is categorized into three different regions which cross over each other. Among them, the weak-coupling region exhibits spatially extended Cooper pairs, which are nevertheless distinct from the conventional pairing of two electrons with opposite momenta. 71.23.Ft, 67.85.Lm Quasicrystal is a crystal without translational symmetry. Prominent spots observed in its diffraction pattern manifest an orderly structure while they do not conform to any periodicity. An example of such structures holds the icosahedral pointgroup symmetry, as first discovered by Shechtman et al. [1], and various other structures have hitherto been reported [2][3][4]. These structures may originate novel electronic properties distinct from those of conventional periodic crystals. In fact, previous theoretical works revealed various nontrivial properties, such as the presence of a confined state [5,6], fractal dimensions [7][8][9], singular continuous spectral measure [8,10,11], pseudogap in the density of states [12], and a conductance decaying in power of system size [13,14], for free electrons on the quasiperiodic lattices. Moreover, recent observation of quantum critical behavior in Au 51 Al 34 Yb 15 [15] has stimulated theoretical studies [16][17][18][19][20][21][22] on the role of electron correlations in these systems.Another interesting recent observation is a superconductivity in approximant crystals (i.e., periodic crystals with the same local structure as the quasicrystals), Au 64 Ge 22 Yb 14 and Au 63.5 Ge 20.5 Yb 16 [23]. A superconductivity has also been reported in Al-Cu-(Mg, Li) quasicrystalline alloys [24,25]. These observations raise fundamental questions about a possible superconductivity in quasicrystals: How can a superconductivity emerge in a system without translational symmetry? If it exists, what differs from the superconductivity in periodic systems? These questions also have a relevance to experiment of ultracold atomic gases, for which optical quasiperiodic lattices have been available [26][27][28].According to an early consideration by Anderson [29] about the impurity effect on superconductivity, Cooper pairs can exist in principle even in the absence of the translational symmetry. In this case, an electron finds its partner in the time-reversed state, which is a generalization of the standard pairing of k ↑ and −k ↓. However, as a matter of course, this does not guarantee the presence of superconductivity in quasiperiodic systems. This many-body problem requires an explicit calculation taking into account both the pairing interaction and the lattice geome...

The thermoelectric effect is attracting a renewed interest as a concept for energy harvesting technologies. Nanomaterials have been considered a key to realize efficient thermoelectric conversions owing to the low dimensional charge and phonon transports. In this regard, recently emerging two-dimensional materials could be promising candidates with novel thermoelectric functionalities. Here we report that FeSe ultrathin films, a high-Tc superconductor (Tc; superconducting transition temperature), exhibit superior thermoelectric responses. With decreasing thickness d, the electrical conductivity increases accompanying the emergence of high-Tc superconductivity; unexpectedly, the Seebeck coefficient α shows a concomitant increase as a result of the appearance of two-dimensional natures. When d is reduced down to ~1 nm, the thermoelectric power factor at 50 K and room temperature reach unprecedented values as high as 13,000 and 260 μW cm−1 K−2, respectively. The large thermoelectric effect in high Tc superconductors indicates the high potential of two-dimensional layered materials towards multi-functionalization.

Molecular solids with cooperative electronic properties based purely on π electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of high-temperature superconductivity in materials obtained by the reaction of alkali metals with polyaromatic hydrocarbons, such as phenanthrene and picene, but the composition and structure of any compound in this family remained unknown. Here we isolate the binary caesium salts of phenanthrene, Cs(CH) and Cs(CH), to show that they are multiorbital strongly correlated Mott insulators. Whereas Cs(CH) is diamagnetic because of orbital polarization, Cs(CH) is a Heisenberg antiferromagnet with a gapped spin-liquid state that emerges from the coupled highly frustrated Δ-chain magnetic topology of the alternating-exchange spiral tubes of S = ½ (CH) radical anions. The absence of long-range magnetic order down to 1.8 K (T/J ≈ 0.02; J is the dominant exchange constant) renders the compound an excellent candidate for a spin-½ quantum-spin liquid (QSL) that arises purely from carbon π electrons.

We study the one-band Hubbard model on the honeycomb lattice using a combination of quantum Monte Carlo (QMC) simulations and static as well as dynamical mean-field theory (DMFT). This model is known to show a quantum phase transition between a Dirac semi-metal and the antiferromagnetic insulator. The aim of this article is to provide a detailed comparison between these approaches by computing static properties, notably ground-state energy, single-particle gap, double occupancy, and staggered magnetization, as well as dynamical quantities such as the single-particle spectral function. At the static mean-field level local moments cannot be generated without breaking the SU(2) spin symmetry. The DMFT approximation accounts for temporal fluctuations and captures the local moment formation in the paramagnetic phase. As a consequence, the DMFT approximation is found to be very accurate in the Dirac semi-metallic phase where local moment formation is present and the spin correlation length small. However, in the vicinity of the fermion quantum critical point the spin correlation length diverges and the spontaneous SU(2) symmetry breaking leads to low-lying Goldstone modes in the magnetically ordered phase. The impact of these spin fluctuations on the single particle spectral function -waterfall features and narrow spin-polaron bands -is only visible in the lattice QMC approach. arXiv:1908.04307v1 [cond-mat.str-el]