354 research outputs found
Dynamics vs electronic states of vortex core of high-T_c superconductors investigated by high-frequency impedance measurement
Dynamics of vortices reflects the electronic states of quasiparticles in the
core. To understand this, we investigated the following three issues. (1) We
investigated the complex surface impedance, Zs, of YBa2Cu3Oy as a function of
magnetic field, H. The total features were well expressed by the Coffey-Clem
model. From the data, we estimated the viscosity and pinning frequency, which
were found to be independent of frequency. In particular, the obtained
viscosity definitely shows that the core of vortex of YBa2Cu3Oy is moderately
clean. This result suggests that new physics will show up, for the physics of
quantum moderately clean vortex core is unknown at all. (2) An anomaly found in
the surface reactance at the first order transition (FOT) of vortex lattice was
investigated in Bi2Sr2CaCu2Oy with various doping levels. As a result, the
anomaly was found only in the samples exhibiting the FOT. On the other hand, we
did not observe the anomaly in YBa2Cu3Oy. These suggest that the anomaly is due
to the change in the electronic states of the vortices characteristic of
materials with very strong anisotropy. (3) We measured H dependence of both the
thermal conductivity \kappa(H) and Zs(H) in exactly the same pieces of crystal.
We could not find any anomaly in Zs(H) even at the onset of the plateau. This
result suggests that the origin of the plateau in \kappa(H) is not a drastic
phase transition but is rather gradual crossover.Comment: 6 pages, 5 figures, Proceedings of Plasma2000(Sendai), to be
published in Physica
Searching for Majorana quasiparticles at vortex cores in iron-based superconductors
The unambiguous detection of the Majorana zero mode (MZM), which is essential
for future topological quantum computing, has been a challenge in recent
condensed matter experiments. The MZM is expected to emerge at the vortex core
of topological superconductors as a zero-energy vortex bound state (ZVBS),
amenable to detection using scanning tunneling microscopy/spectroscopy
(STM/STS). However, the typical energy resolution of STM/STS has made it
challenging to distinguish the MZM from the low-lying trivial vortex bound
states. Here, we review the recent high-energy-resolution STM/STS experiments
on the vortex cores of Fe(Se,Te), where the MZM is expected to emerge, and the
energy of the lowest trivial bound states is reasonably high. Tunneling spectra
taken at the vortex cores exhibit a ZVBS well below any possible trivial state,
suggesting its MZM origin. However, it should be noted that ZVBS is a necessary
but not sufficient condition for the MZM; a qualitative feature unique to the
MZM needs to be explored. We discuss the current status and issues in the
pursuit of such Majorananess, namely the level sequence of the vortex bound
states and the conductance plateau of the ZVBS. We also argue for future
experiments to confirm the Majorananess, such as the detection of the doubling
of the shot noise intensity and spin polarization of the MZM.Comment: 22 pages, 7 figure
Observation of Zeeman effect in topological surface state with distinct material dependence
The helical Dirac fermions on the surface of topological insulators host
novel relativistic quantum phenomena in solids. Manipulating spins of
topological surface state (TSS) represents an essential step towards exploring
the theoretically predicted exotic states related to time reversal symmetry
(TRS) breaking via magnetism or magnetic field. Understanding Zeeman effect of
TSS and determining its g-factor are pivotal for such manipulations in the
latter form of TRS breaking. Here, we report those direct experimental
observations in Bi2Se3 and Sb2Te2Se by spectroscopic imaging scanning tunneling
microscopy. The Zeeman shifting of zero mode Landau level is identified
unambiguously by judiciously excluding the extrinsic influences associated with
the non-linearity in the TSS band dispersion and the spatially varying
potential. The g-factors of TSS in Bi2Se3 and Sb2Te2Se are determined to be 18
and -6, respectively. This remarkable material dependence opens a new route to
control the spins in the TSS.Comment: main text: 17 pages, 4 figures; supplementary: 15 pages, 7 figure
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