We report systematic optical studies of WS2 and WSe2 monolayers and multilayers. The efficiency of second harmonic generation shows a dramatic even-odd oscillation with the number of layers, consistent with the presence (absence) of inversion symmetry in even-layer (odd-layer). Photoluminescence (PL) measurements show the crossover from an indirect band gap semiconductor at multilayers to a direct-gap one at monolayers. A hot luminescence peak (B) is observed at ~0.4 eV above the prominent band edge peak (A) in all samples. The magnitude of A-B splitting is independent of the number of layers and coincides with the spin-valley coupling strength in monolayers. Ab initio calculations show that this thickness independent splitting pattern is a direct consequence of the giant spin-valley coupling which fully suppresses interlayer hopping at valence band edge near K points because of the sign change of the spin-valley coupling from layer to layer in the 2H stacking order.

Single-layer FeSe film on SrTiO3 (001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle interference (QPI) patterns reveal scatterings between and within the electron pockets, and put constraints on possible pairing symmetries. By introducing impurity atoms onto the sample, we show that the magnetic impurities (Cr, Mn) can locally suppress the superconductivity but the non-magnetic impurities (Zn, Ag and K) cannot. Our results indicate that single-layer FeSe/ SrTiO3 has a plain s-wave paring symmetry whose order parameter has the same phase on all Fermi surface sections.Recently the discovery of enhanced superconductivity in single-layer FeSe on SrTiO3(001) has attracted tremendous interest [1][2][3][4][5][6][7][8][9], not only for the new possible superconducting transition temperature records of Fe-based superconductors and interfacial superconductors (65K [3,4] or even higher [9]), but also its intriguing mechanism that enhances the paring. Thus it is of great importance to understand the pairing symmetry and underlying electron structure of single-layer FeSe/SrTiO3(001). Angle-resolved photoemission spectroscopy (ARPES) revealed that such films have only electron Fermi surfaces, similar to that of the alkali metal intercalated iron selenides (AxFe2-ySe2, A=K, Cs…) [3][4][5]. This seriously challenges the original s±-pairing scenario proposed for the iron pnictides that relies on the coupling between the electron pockets and the hole pockets at the Brillouin zone center [10,11]. Meanwhile both ARPES and previous STM studies found fully gapped superconducting state in single-layer FeSe, indicative of the absence of gap nodes [1,[3][4][5]. Various possible paring symmetries have been proposed for such systems with only electron pockets [12][13][14][15][16][17][18][19], such as plain s-wave paring [12][13][14], "quasi-nodeless" d-wave paring [15,16], and several new types of s± paring that involve the "folding" of Brillouin zone and band hybridization [17], orbital dependent pairing [18], or mixing of the even and odd-parity pairing [19]. Except the plain s-wave paring, all the other proposed pairing symmetries involve sign changing of the order parameter on different sections of the Fermi surface. To distinguish these scenarios, phase sensitive measurements are required, plus the detailed knowledge on the superconducting gap.STM has been shown to be able to provide information on the pairing symmetry by measuring local response of superconductivity to impurities (in-gap impurity states) [20][21][22] and throu...

We report that sizable single crystals of BaFe2As2 have been grown with the self-flux method. Measurements and anisotropy of intrinsic transport and magnetic properties from high-quality single crystal are first presented. The resistivity anisotropy (rho{c}/rho{ab}) is as large as 150 and independent of temperature. In contrast to the susceptibility behavior observed in polycrystalline samples, no Curie-Weiss behavior is observed, and a linear-T dependent susceptibility occurs from the spin-density-wave transition temperature, (T{s}), to 700 K. This result suggests that strong antiferromagnetic correlations are present well above T{s}. A twofold symmetry of susceptibility in the ab plane indicates a stripelike spin structure as observed by neutron scattering. The resistivity minimum is strongly dependent on the magnetic field, suggesting that the upturn of the resistivity at low temperatures should be related to spin fluctuation.

The recent discovery of superconductivity in oxypnictides with a critical transition temperature (T(C)) higher than the McMillan limit of 39 K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer theory) has generated great excitement. Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures, but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible. Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotope substitution on T(C) and the spin-density wave (SDW) transition temperature (T(SDW)) in the SmFeAsO(1 - x)F(x) and Ba(1 - x)K(x)Fe(2)As(2) systems. The oxygen isotope effect on T(C) and T(SDW) is very small, while the iron isotope exponent alpha(C) = -dlnT(C)/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on T(SDW) as T(C). This indicates that electron-phonon interaction plays some role in the superconducting mechanism, but a simple electron-phonon coupling mechanism seems unlikely because a strong magnon-phonon coupling is included.

Sr 2 IrO 4 was predicted to be a high-temperature superconductor upon electron doping since it highly resembles the cuprates in crystal structure, electronic structure, and magnetic coupling constants. Here, we report a scanning tunneling microscopy/spectroscopy (STM/STS) study of Sr 2 IrO 4 with surface electron doping by depositing potassium (K) atoms. We find that as the electron doping increases, the system gradually evolves from an insulating state to a normal metallic state, via a pseudogaplike phase, and a phase with a sharp, V-shaped low-energy gap with about 95% loss of density of state (DOS) at E F . At certain K coverage (0.5-0.6 monolayer), the magnitude of the low-energy gap is 25-30 meV, and it closes at around 50 K. Our observations show that the electron-doped Sr 2 IrO 4 remarkably resembles hole-doped cuprate superconductors.

The Majorana fermion, which is its own anti-particle and obeys non-abelian statistics, plays a critical role in topological quantum computing. It can be realized as a bound state at zero energy, called a Majorana zero mode (MZM), in the vortex core of a topological superconductor, or at the ends of a nanowire when both superconductivity and strong spin orbital coupling are present. A MZM can be detected as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. However, in practice, clean and robust MZMs have not been realized in the vortices of a superconductor, due to contamination from impurity states or other closely-packed Caroli-de Gennes-Matricon (CdGM) states, which hampers further manipulations of MZMs. Here using scanning tunneling spectroscopy, we show that a ZBCP well separated from the other discrete CdGM states exists ubiquitously in the cores of free vortices in the defect free regions of (Li0.84Fe0.16)OHFeSe, which has a superconducting transition temperature of 42 K. Moreover, a Dirac-cone-type surface state is observed by angle-resolved photoemission spectroscopy, and its topological nature is confirmed by band calculations. The observed ZBCP can be naturally attributed to a MZM arising from this chiral topological surface states of a bulk superconductor. (Li0.84Fe0.16)OHFeSe thus provides an ideal platform for studying MZMs and topological quantum computing. 2

We synthesized the samples Sr(1-x)Sm(x)FFeAs with a ZrCuSiAs-type structure. These samples were characterized by resistivity and susceptibility. It is found that substitution of rare earth metal for alkaline earth metal in this system suppresses the anomaly in resistivity and induces superconductivity. Superconductivity at 56 K in nominal composition Sr(0.5)Sm(0.5)FFeAs is realized, indicating that the superconducting transition temperatures in the iron arsenide fluorides can reach as high as that in oxypnictides with the same structure.

Using angle-resolved photoemission spectroscopy (ARPES), we revealed the surface electronic structure and superconducting gap of (Li 0.8 Fe 0.2 )OHFeSe, an intercalated FeSe-derived superconductor without antiferromagnetic phase or Fe-vacancy order in the FeSe layers, and with a superconducting transition temperature (T c ) ∼ 40 K. We found that (Li 0.8 Fe 0.2 )OH layers dope electrons into FeSe layers. The electronic structure of surface FeSe layers in (Li 0.8 Fe 0.2 )OHFeSe resembles that of Rb x Fe 2−y Se 2 except that it only contains half of the carriers due to the polar surface, suggesting similar quasiparticle dynamics between bulk (Li 0.8 Fe 0.2 )OHFeSe and Rb x Fe 2−y Se 2 . Superconducting gap is clearly observed below T c , with an isotropic distribution around the electron Fermi surface. Compared with A x Fe 2−y Se 2 (A=K, Rb, Cs, Tl/K), the higher T c in (Li 0.8 Fe 0.2 )OHFeSe might be attributed to higher homogeneity of FeSe layers or to some unknown roles played by the (Li 0.8 Fe 0.2 )OH layers.