441 research outputs found
Anomalously enhanced photoemission from the Dirac point and symmetry of the self-energy variations for the surface states in Bi2Se3
Accurate analysis of the photoemission intensity from the surface states of
Bi2Se3 reveals two unusual features: spectral line asymmetry and anomalously
enhanced photoemission from the Dirac point. The former indicates a certain
symmetry of a scattering process, which results in strongly k\omega-dependent
contribution to the imaginary part of the self-energy that changes sign while
crossing both the dispersion curves and the energy of the Dirac point. The
latter is hard to describe by one particle spectral function while a final
state interference seems to be plausible explanation
Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2
We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice
relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped
Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1}
is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain.
Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in
the Ca-doped sample evidencing the opening of a spin gap. The data analysis
within the J_1-J_2 Heisenberg model employing density-matrix renormalization
group calculations suggests an impurity driven small alternation of the
J_2-exchange coupling as a possible cause of the spin gap.Comment: 4 pages, 4 figure
Photoemission induced gating of topological insulator
The recently discovered topological insulators exhibit topologically
protected metallic surface states which are interesting from the fundamental
point of view and could be useful for various applications if an appropriate
electronic gating can be realized. Our photoemission study of Cu intercalated
Bi2Se3 shows that the surface states occupancy in this material can be tuned by
changing the photon energy and understood as a photoemission induced gating
effect. Our finding provides an effective tool to investigate the new physics
coming from the topological surface states and suggests the intercalation as a
recipe for synthesis of the material suitable for electronic applications.Comment: + resistivity data and some discussio
Signatures of superconducting and pseudogap phases in ultrafast transient reflectivity of Ca(Fe0.927Co0.073)2As2
We present femtosecond time-resolved pump-probe spectroscopic studies of a
pseudogap (PG) along with the superconducting (SC) gap in an overdoped iron
pnictide Ca(Fe0.927Co0.073)2As2. It is seen that the temperature evolution of
the photoexcited quasiparticle (QP) relaxation dynamics, coherently excited
A1g-symmetric optical phonon and two acoustic phonon dynamics behave
anomalously in the vicinity of the superconducting transition temperature Tc. A
continuous change in the sign of the experimentally measured transient
differential reflectivity Delta_R/R signal at the zero time-delay between the
pump and probe pulses at a temperature of ~200 K is inferred as an evidence of
the emergence of the PG phase around that temperature. This behavior is
independent of the pump photon energy and occurs for crystals without the spin
density wave phase transition.Comment: EPL (2014
Magnetic excitations in two-leg spin 1/2 ladders: experiment and theory
Magnetic excitations in two-leg S=1/2 ladders are studied both experimentally
and theoretically. Experimentally, we report on the reflectivity, the
transmission and the optical conductivity sigma(omega) of undoped La_x Ca_14-x
Cu_24 O_41 for x=4, 5, and 5.2. Using two different theoretical approaches
(Jordan-Wigner fermions and perturbation theory), we calculate the dispersion
of the elementary triplets, the optical conductivity and the momentum-resolved
spectral density of two-triplet excitations for 0.2 <=
J_parallel/J_perpendicular <= 1.2. We discuss phonon-assisted two-triplet
absorption, the existence of two-triplet bound states, the two-triplet
continuum, and the size of the exchange parameters.Comment: 6 pages, 7 eps figures, submitted to SNS 200
- …