We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Se-etched SrTiO3(001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising T-C, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.
We report the superconductivity in iron-based oxyarsenide Sm[O1-xFx]FeAs, with the onset resistivity transition temperature at 55.0K and Meissner transition at 54.6 K. This compound has the same crystal structure as LaOFeAs with shrunk crystal lattices, and becomes the superconductor with the highest critical temperature among all materials besides copper oxides up to now.
The discovery of cuprate high T-C superconductors has inspired the search for unconventional superconductors in magnetic materials. A successful recipe has been to suppress long-range order in a magnetic parent compound by doping or high pressure to drive the material towards a quantum critical point. We report an exception to this rule in the recently discovered potassium iron selenide. The superconducting composition is identified as the iron vacancy ordered K0.83(2)Fe1.64(1)Se2 with T-C above 30K. A novel large moment 3.31 mu(B)/Fe antiferromagnetic order that conforms to the tetragonal crystal symmetry has an unprecedentedly high ordering temperature T-N approximate to 559 K for a bulk superconductor. Staggeringly polarized electronic density of states is thus suspected, which would stimulate further investigation into superconductivity in a strong spin-exchange field under new circumstances.
The organic-inorganic hybrid perovskite CH3NH3PbI3 has attracted significant interest for its high performance in converting solar light into electrical power with an efficiency exceeding 20%. Unfortunately, chemical stability is one major challenge in the development of CH3NH3PbI3 solar cells. It was commonly assumed that moisture or oxygen in the environment causes the poor stability of hybrid halide perovskites, however, here we show from the first-principles calculations that the room-temperature tetragonal phase of CH3NH3PbI3 is thermodynamically unstable with respect to the phase separation into CH3NH3I + PbI2, i.e., the disproportionation is exothermic, independent of the humidity or oxygen in the atmosphere. When the structure is distorted to the low-temperature orthorhombic phase, the energetic cost of separation increases, but remains small. Contributions from vibrational and configurational entropy at room temperature have been considered, but the instability of CH3NH3PbI3 is unchanged. When I is replaced by Br or Cl, Pb by Sn, or the organic cation CH3NH3 by inorganic Cs, the perovskites become more stable and do not phase-separate spontaneously. Our study highlights that the poor chemical stability is intrinsic to CH3NH3PbI3 and suggests that element-substitution may solve the chemical stability problem in hybrid halide perovskite solar cells.
We prepared one-unit-ceil （1-UC） thick FeSe films on insulating SrTiOa substrates with non-superconducting FeTe protection layers by molecular beam epitaxy for ex situ studies. By direct transport and magnetic measurements, we provide definitive evidence for high temperature superconductivity in the 1-UC FeSe films with an onset Tc above 40 K and an extremely large critical current density fie Jc-1.7× 106 A/cm2 at 2K, which are much higher than Tc-8K and Jc-104 A/cm^2 for bulk FeSe, respectively. Our work may pave the way to enhancing and tailoring superconductivity by interface engineering.
According to the "no-node" theorem, the many-body ground state wavefunctions of conventional Bose-Einstein condensations (BEC) are positive-definite, thus time-reversal symmetry cannot be spontaneously broken. We find that multi-component bosons with spin-orbit coupling provide an unconventional type of BECs beyond this paradigm. We focus on a subtle case of isotropic Rashba spin-orbit coupling and the spin-independent interaction. In the limit of the weak confining potential, the condensate wavefunctions are frustrated at the Hartree-Fock level due to the degeneracy of the Rashba ring. Quantum zero-point energy selects the spin-spiral type condensate through the "order-from-disorder" mechanism. In a strong harmonic confining trap, the condensate spontaneously generates a half-quantum vortex combined with the skyrmion type of spin texture. In both cases, time-reversal symmetry is spontaneously broken. These phenomena can be realized in both cold atom systems with artificial spin-orbit couplings generated from atom-laser interactions and exciton condensates in semi-conductor systems.
We utilize a new framework, CUJET3.0, to deduce the energy and temperature dependence of the jet trans- port parameter, q （E 〉 10 GeV, T）, from a combined analysis of available data on nudear modification factor and azimuthal asymmetries from high energy nuclear collisions at RHIC/BNL and LHC/CERN. Extending a previous perturbative-QCD based jet energy loss model （known as CUJET2.0） with （2＋1）D viscous hydrody- namic bulk evolution, this new framework includes three novel features of nonperturbative physics origin; （i） the Polyakov loop suppression of color-electric scattering （aka ＇semi-QGP＇ of Pisarski et al.）, （ii） the enhancement of jet scattering due to emergent magnetic monopoles near Tc （aka ＇magnetic scenario＇ of Liao and Shuryak）, and Oii） thermodynamic properties constrained by lattice QCD data. CUJET3.0 reduces to v2.0 at high tem- peratures T 〉400 MeV, while greatly enhances ~ near the QCD deconfinement transition temperature range. This enhancement accounts well for the observed elliptic harmonics of jets with pT 〉10 GeV. Extrapolating our data-constrained q down to thermal energy scales, E-2 GeV, we find for the first time a remarkable consistency between high energy jet quenching and bulk perfect fluidity with η/s~T3/q~0.1 near To.
A series of layered (Sr1-xKx)Fe2As2 compounds with nominal x = 0-0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron- based pnictide superconductors, pure SrFe2As2 shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe2As2, the two prototype parent compounds with ZrCuSiAs- and ThCr2Si2- type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case of K-doped BaFe2As2, sharp superconducting transitions at T-c similar to 38K is observed. We perform the Hall coefficient measurement, and confirm that the dominant carriers are hole-type. The carrier density is enhanced by a factor of 3 in comparison to F-doped LaOFeAs superconductor.
Recently, the concept of topological insulators has been generalized to topological semimetals, including three-dimensional （3D） Weyl semimetals, 3D Dirac semimetMs, and 3D node-line semimetals （NLSs）. In particular, several compounds （e.g., certain 3D graphene networks, Cu3PdN, Ca3P2 ） were discovered to be 3D NLSs, in which the conduction and valence bands cross at closed lines in the Brillouin zone. Except for the two-dimensional （2D） Dirac semimetal （e.g., graphene）, 2D topological semimetals are much less investigated. Here we propose a new concept of a 2D NLS and suggest that this state could be realized in a new mixed lattice （named as HK lattice） composed by Kagome and honeycomb lattices. It is found that A3B2 （A is a group-liB cation and B is a group-VA anion） compounds （such as Hg3As2） with the HK lattice are 2D NLSs due to the band inversion between the cation Hg-s orbital and the anion As-pz orbital with respect to the mirror symmetry. Since the band inversion occurs between two bands with the same parity, this peculiar 2D NLS could be used as transparent conductors. In the presence of buckling or spin-orbit coupling, the 2D NLS state may turn into a 2D Dirac semimetal state or a 2D topological crystalline insulating state. Since the band gap opening due to buckling or spin-orbit coupling is small, Hg3As3 with the HK lattice can still be regarded as a 2D NLS at room temperature. Our work suggests a new route to design topological materials without involving states with opposite parities.
We report the surface electronic structure of niobium phosphide NbP single crystal on （001） surface by vacuum ultraviolet angle-resolved photoemission spectroscopy. Combining with our first principle calculations, we identify the existence of the Fermi arcs originated from topological surface states. Furthermore, the surface states exhibit circular dichroism pattern, which may correlate with its non-trivial spin texture. Our results provide critical evidence for the existence of the Weyl Fermions in NbP, which lays the foundation for further research.
We report a new kagome quantum spin liquid candidate Cu3Zn(OH)(6)FBr, which does not experience any phase transition down to 50 mK, more than three orders lower than the antiferromagnetic Curie-Weiss temperature (similar to 200 K). A clear gap opening at low temperature is observed in the uniform spin susceptibility obtained from F-19 nuclear magnetic resonance measurements. We observe the characteristic magnetic field dependence of the gap as expected for fractionalized spin-1/2 spinon excitations. Our experimental results provide firm evidence for spin fractionalization in a topologically ordered spin system, resembling charge fractionalization in the fractional quantum Hall state.
The mixed convection stagnation-point flow of an incompressible non-Newtonian fluid over a stretching sheet under convective boundary conditions is investigated. Mathematical formulation is presented for a Casson fluid. The resulting partial differential equations are converted into the ordinary differential equations by the suitable transformations. The velocity and temperature profiles are computed by employing the homotopy analysis method. The plotted graphs illustrate the flow and heat transfer characteristics and their dependence upon the embedded parameters. Numerical values of skin-friction coefficient and Nusselt number are given and examined. Comparison of the present results with the existing solution is also given.
Breathers and rogue waves as exact solutions of the three-dimensional Kadomtsev Petviashvili equation are obtained via the bilinear transformation method. The breathers in three dimensions possess different dynamics in different planes, such as growing and decaying periodic line waves in the （x, y）, （x, z） and （y, t） planes. Rogue waves are localized in time, and are obtained theoretically as a long wave limit of breathers with indefinitely larger periods. It is shown that the rogue waves possess growing and decaying line profiles in the （x, y） or （x, z） plane, which arise from a constant background and then retreat back to the same background again.
The (2+1)-dimension nonlocal nonlinear Schrodinger (NLS) equation with the self-induced parity-time symmetric potential is introduced, which provides spatially two-dimensional analogues of the nonlocal NLS equation introduced by Ablowitz et al. [Phys. Rev. Lett. 110 (2013) 064105]. General periodic solutions are derived by the bilinear method. These periodic solutions behave as growing and decaying periodic line waves arising from the constant background and decaying back to the constant background again. By taking long wave limits of the obtained periodic solutions, rogue waves are obtained. It is also shown that these line rogue waves arise from the constant background with a line profile and disappear into the constant background again in the (x, y) plane.
The total reciprocal space magnetic flux threading through a closed Fermi surface is a topological invariant for a three-dimensional metal. For a Weyl metal, the invariant is nonzero for each of its Fermi surfaces. We show that such an invariant can be related to the magneto-valley-transport effect, in which an external magnetic field can induce a valley current. We further show that a strain field can drive an electric current, and that the effect is dictated by a second-class Chern invariant. These connections open the pathway to observe the hidden topological invariants in metallic systems.