A Measurement of the Tau Hadronic Branching Ratios

The exclusive and semi-exclusive branching ratios of the tau lepton hadronic decay modes (h- v_t, h- pi0 v_t, h- pi0 pi0 v_t, h- \geq 2pi0 v_t, h- \geq 3pi0 v_t, 2h- h+ v_t, 2h- h+ pi0 v_t, 2h- h+ \geq 2pi0 v_t, 3h- 2h+ v_t and 3h- 2h+ \geq 1pi0 v_t) were measured with data from the DELPHI detector at LEP.Comment: 53 pages, 18 figures, Accepted by Eur. Phys. J.

Orbital selectivity causing anisotropy and particle-hole asymmetry in the charge density wave gap of 2H2H-TaS2_2

We report an in-depth Angle Resolved Photoemission Spectroscopy (ARPES) study on $2H$-TaS$_2$, a canonical incommensurate Charge Density Wave (CDW) system. This study demonstrates that just as in related incommensurate CDW systems, $2H$-TaSe$_2$ and $2H$-NbSe$_2$, the energy gap ($\Delta_{\text{cdw}}\,$) of $2H$-TaS$_2$ is localized along the K-centered Fermi surface barrels and is particle-hole asymmetric. The persistence of $\Delta_{\text{cdw}}\,$ even at temperatures higher than the CDW transition temperature $\it{T}_{\text{cdw}}\,$ in $2H$-TaS$_2$, reflects the similar pseudogap (PG) behavior observed previously in $2H$-TaSe$_2$ and $2H$-NbSe$_2$. However, in sharp contrast to $2H$-NbSe$_2$, where $\Delta_{\text{cdw}}\,$ is non-zero only in the vicinity of a few "hot spots" on the inner K-centered Fermi surface barrels, $\Delta_{\text{cdw}}\,$ in $2H$-TaS$_2$ is non-zero along the entirety of both K-centered Fermi surface barrels. Based on a tight-binding model, we attribute this dichotomy in the momentum dependence and the Fermi surface specificity of $\Delta_{\text{cdw}}\,$ between otherwise similar CDW compounds to the different orbital orientations of their electronic states that are involved in CDW pairing. Our results suggest that the orbital selectivity plays a critical role in the description of incommensurate CDW materials.Comment: 6 pages, 4 figure

Amplitude `Higgs' mode in 2H-NbSe2 Superconductor

We report experimental evidences for the observation of the superconducting amplitude mode, so-called `Higgs' mode in the charge density wave superconductor 2H-NbSe2 using Raman scattering. By comparing 2H-NbSe2 and its iso-structural partner 2H-NbS2 which shows superconductivity but lacks the charge density wave order, we demonstrate that the superconducting mode in 2H-NbSe2 owes its spectral weight to the presence of the coexisting charge density wave order. In addition, temperature dependent measurements in 2H-NbSe2 show a full spectral weight transfer from the charge density wave mode to the superconducting mode upon entering the superconducting phase. Both observations are fully consistent with a superconducting amplitude mode or Higgs mode.Comment: Accepted for publication in Phys. Rev. B Rapid Com. 5 pages with 3 figure

Strong enhancement of superconductivity at high pressures within the charge-density-wave states of 2H-TaS 2 and 2H-TaSe 2

We present measurements of the superconducting and charge density wave critical temperatures (Tc and TCDW) as a function of pressure in the transition metal dichalchogenides 2H-TaSe2 and 2H-TaS2. Resistance and susceptibility measurements show that Tc increases from temperatures below 1 K up to 8.5 K at 9.5 GPa in 2H-TaS2 and 8.2 K at 23 GPa in 2H-TaSe2. We observe a kink in the pressure dependence of TCDW at about 4 GPa that we attribute to the lock-in transition from incommensurate CDW to commensurate CDW. Above this pressure, the commensurate TCDW slowly decreases coexisting with superconductivity within our full pressure range.Comment: Published in Phys. Rev B 93, 184512 (2016

Magnetism in Semiconducting Molybdenum Dichalcogenides

Transition metal dichalcogenides (TMDs) are interesting for understanding fundamental physics of two-dimensional materials (2D) as well as for many emerging technologies, including spin electronics. Here, we report the discovery of long-range magnetic order below TM = 40 K and 100 K in bulk semiconducting TMDs 2H-MoTe2 and 2H-MoSe2, respectively, by means of muon spin-rotation (muSR), scanning tunneling microscopy (STM), as well as density functional theory (DFT) calculations. The muon spin rotation measurements show the presence of a large and homogeneous internal magnetic fields at low temperatures in both compounds indicative of long-range magnetic order. DFT calculations show that this magnetism is promoted by the presence of defects in the crystal. The STM measurements show that the vast majority of defects in these materials are metal vacancies and chalcogen-metal antisites which are randomly distributed in the lattice at the sub-percent level. DFT indicates that the antisite defects are magnetic with a magnetic moment in the range of 0.9-2.8 mu_B. Further, we find that the magnetic order stabilized in 2H-MoTe2 and 2H-MoSe2 is highly sensitive to hydrostatic pressure. These observations establish 2H-MoTe2 and 2H-MoSe2 as a new class of magnetic semiconductors and opens a path to studying the interplay of 2D physics and magnetism in these interesting semiconductors.Comment: 13 pages, 10 Figure

Coexistence of gapless excitations and commensurate charge-density wave in the 2H-transition metal dichalcogenides

An unexpected feature common to 2H-transition metal dichalcogenides (2H-TMDs) is revealed with first-principles Wannier functions analysis of the electronic structure of the prototype 2H-TaSe2: The low-energy Ta \red{``$5d_{z^2}$''} bands governing the physics of charge-density wave (CDW) is dominated by hopping between next-nearest neighbors. With this motivation we develop a minimal effective model for the CDW formation, in which the unusual form of the hopping leads to an approximate decoupling of the three sublattices. In the CDW phase one sublattice remains undistorted, leaving the bands associated with it ungapped everywhere in the Fermi surface, resolving the long-standing puzzle of coexistence of gapless excitations and commensurate CDW in the 2H-TMDs.Comment: 4 pages, 5 figure