314 research outputs found
A topological insulator surface under strong Coulomb, magnetic and disorder perturbations
Three dimensional topological insulators embody a newly discovered state of
matter characterized by conducting spin-momentum locked surface states that
span the bulk band gap as demonstrated via spin-resolved ARPES measurements .
This highly unusual surface environment provides a rich ground for the
discovery of novel physical phenomena. Here we present the first controlled
study of the topological insulator surfaces under strong Coulomb, magnetic and
disorder perturbations. We have used interaction of iron, with a large Coulomb
state and significant magnetic moment as a probe to \textit{systematically test
the robustness} of the topological surface states of the model topological
insulator BiSe. We observe that strong perturbation leads to the
creation of odd multiples of Dirac fermions and that magnetic interactions
break time reversal symmetry in the presence of band hybridization. We also
present a theoretical model to account for the altered surface of BiSe.
Taken collectively, these results are a critical guide in manipulating
topological surfaces for probing fundamental physics or developing device
applications.Comment: 14 pages, 4 Figures. arXiv admin note: substantial text overlap with
arXiv:1009.621
A Low Temperature Nonlinear Optical Rotational Anisotropy Spectrometer for the Determination of Crystallographic and Electronic Symmetries
Nonlinear optical generation from a crystalline material can reveal the
symmetries of both its lattice structure and underlying ordered electronic
phases and can therefore be exploited as a complementary technique to
diffraction based scattering probes. Although this technique has been
successfully used to study the lattice and magnetic structures of systems such
as semiconductor surfaces, multiferroic crystals, magnetic thin films and
multilayers, challenging technical requirements have prevented its application
to the plethora of complex electronic phases found in strongly correlated
electron systems. These requirements include an ability to probe small bulk
single crystals at the micron length scale, a need for sensitivity to the
entire nonlinear optical susceptibility tensor, oblique light incidence
reflection geometry and incident light frequency tunability among others. These
measurements are further complicated by the need for extreme sample
environments such as ultra low temperatures, high magnetic fields or high
pressures. In this review we present a novel experimental construction using a
rotating light scattering plane that meets all the aforementioned requirements.
We demonstrate the efficacy of our scheme by making symmetry measurements on a
micron scale facet of a small bulk single crystal of SrIrO using
optical second and third harmonic generation.Comment: 8 pages, 5 figure
Topologically non-trivial magnon bands in artificial square spin ices subject to Dzyaloshinskii-Moriya interaction
Systems that exhibit topologically protected edge states are interesting both
from a fundamental point of view as well as for potential applications, the
latter because of the absence of back-scattering and robustness to
perturbations. It is desirable to be able to control and manipulate such edge
states. Here, we show that artificial square ices can incorporate both
features: an interfacial Dzyaloshinksii-Moriya gives rise to topologically
non-trivial magnon bands, and the equilibrium state of the spin ice is
reconfigurable with different configurations having different magnon
dispersions and topology. The topology is found to develop as odd-symmetry bulk
and edge magnon bands approach each other, so that constructive band inversion
occurs in reciprocal space. Our results show that topologically protected bands
are supported in square spin ices.Comment: 27 pages, 6 figure
Nonequilibrium Quasiparticle Relaxation Dynamics in Single Crystals of Hole and Electron doped BaFeAs
We report on the nonequilibrium quasiparticle dynamics in BaFeAs on
both the hole doped (BaKFeAs) and electron doped
(BaFeCoAs) sides of the phase diagram using ultrafast
pump-probe spectroscopy. Below , measurements conducted at low
photoinjected quasiparticle densities in the optimally and overdoped
BaKFeAs samples reveal two distinct relaxation processes: a
fast component whose decay rate increases linearly with excitation density and
a slow component with an excitation density independent decay rate. We argue
that these two processes reflect the recombination of quasiparticles in the two
hole bands through intraband and interband processes. We also find that the
thermal recombination rate of quasiparticles increases quadratically with
temperature in these samples. The temperature and excitation density dependence
of the decays indicates fully gapped hole bands and nodal or very anisotropic
electron bands. At higher excitation densities and lower hole dopings, the
dependence of the dynamics on quasiparticle density disappears as the data are
more readily understood in terms of a model which accounts for the
quasiequilibrium temperature attained by the sample. In the
BaFeCoAs samples, dependence of the recombination rate on
quasiparticle density at low dopings (i.e., ) is suppressed upon
submergence of the inner hole band and quasiparticle relaxation occurs in a
slow, density independent manner.Comment: Accepted to Phys. Rev.
Exchange and collective behavior of magnetic impurities in a disordered helical metal
We study the exchange interaction and the subsequent collective behavior of
magnetic impurities embedded in a disordered two-dimensional (2D) helical
metal. The exchange coupling follows a statistical distribution whose moments
are calculated to the lowest order in , where
is the Fermi momentum of itinerant electrons and is the mean free path.
We find that i) the first moment of the distribution decays exponentially, and
ii) the variance of the interaction is long-range, however, it becomes
independent of the orientation of the localized magnetic moments due to the
locking between spin and momentum of the electrons that mediate the
interaction. As consequence, long-range magnetic order tends to be suppressed,
and a spin glass phase emerges. The formalism is applied to the surface states
of a three-dimensional (3D) topological insulator. The lack of a net magnetic
moment in the glassy phase and the full randomization of spin polarization at
distances larger than excludes a spectral gap for surface states. Hence,
non-magnetic disorder may explain the dispersion in results for photoemission
experiments in magnetically-doped topological insulators.Comment: 5 pages, 3 figures; final version to appear in Physical Review B as a
rapid communicatio
Structure symmetry determination and magnetic evolution in
We use single-crystal neutron diffraction to determine the crystal structure
symmetry and the magnetic evolution in the rhodium doped iridates (). Throughout this doping range, the
crystal structure retains a tetragonal symmetry (space group ) with two
distinct magnetic Ir sites in the unit cell forming staggered
rotation. Upon Rh doping, the magnetic order is suppressed and the magnetic
moment of Ir is reduced from 0.21 /Ir for to 0.18 /Ir for . The magnetic structure at is different from
that of the parent compound while the moments remain in the basal plane.Comment: Accepted for publication in Phys. Rev.
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