43 research outputs found
Chiral charge density wave and backscattering-immune orbital texture in monolayer 1T-TiTe2
Non-trivial electronic states are attracting intense attention in
low-dimensional physics. Though chirality has been identified in charge states
with a scalar order parameter, its intertwining with charge density waves
(CDW), film thickness and the impact on the electronic behaviors remain less
well understood. Here, using scanning tunneling microscopy, we report a 2 x 2
chiral CDW as well as a strong suppression of the Te-5p hole-band
backscattering in monolayer 1T-TiTe2. These exotic characters vanish in bilayer
TiTe2 with a non-CDW state. Theoretical calculations approve that chirality
comes from a helical stacking of the triple-q CDW components and therefore can
persist at the two-dimensional limit. Furthermore, the chirality renders the
Te-5p bands an unconventional orbital texture that prohibits electron
backscattering. Our study establishes TiTe2 as a promising playground for
manipulating the chiral ground states at the monolayer limit and provides a
novel path to engineer electronic properties from an orbital degree.Comment: 21 pages, 5 figure
Magnetic ordering and structural phase transitions in strained ultrathin SrRuO/SrTiO superlattice
Ruthenium-based perovskite systems are attractive because their Structural,
electronic and magnetic properties can be systematically engineered.
SrRuO/SrTiO superlattice, with its period consisting of one unit cell
each, is very sensitive to strain change. Our first-principles simulations
reveal that in the high tensile strain region, it transits from a ferromagnetic
(FM) metal to an antiferromagnetic (AFM) insulator with clear tilted octahedra,
while in the low strain region, it is a ferromagnetic metal without octahedra
tilting. Detailed analyses of three spin-down Ru-t orbitals just below
the Fermi level reveal that the splitting of these orbitals underlies these
dramatic phase transitions, with the rotational force constant of RuO
octahedron high up to 16 meV/Deg, 4 times larger than that of TiO.
Differently from nearly all the previous studies, these transitions can be
probed optically through the diagonal and off-diagonal dielectric tensor
elements. For one percent change in strain, our experimental spin moment change
is -0.140.06 , quantitatively consistent with our theoretical value
of -0.1 .Comment: 3 figures, 1 supplementary material, accepted by Phys. Rev. Let
Spectral signatures of the surface anomalous Hall effect in magnetic axion insulators
The topological surface states of magnetic topological systems, such as Weyl
semimetals and axion insulators, are associated with unconventional transport
properties such as nonzero or half-quantized surface anomalous Hall effect.
Here we study the surface anomalous Hall effect and its spectral signatures in
different magnetic topological phases using both model Hamiltonian and
first-principles calculations. We demonstrate that by tailoring the
magnetization and interlayer electron hopping, a rich three-dimensional
topological phase diagram can be established, including three types of
topologically distinct insulating phases bridged by Weyl semimetals, and can be
directly mapped to realistic materials such as MnBi2Te4/(Bi2Te3)n systems.
Among them, we find that the surface anomalous Hall conductivity in the
axion-insulator phase is a well-localized quantity either saturated at or
oscillating around e2/2h, depending on the magnetic homogeneity. We also
discuss the resultant chiral hinge modes embedded inside the side surface bands
as the potential experimental signatures for transport measurements. Our study
is a significant step forward towards the direct realization of long-sought
axion insulators in realistic material systems.Comment: 22 pages, 4 figure
Ultrafast Suppression of the Ferroelectric Instability in KTaO
We use an x-ray free-electron laser to study the ultrafast lattice dynamics
following above band-gap photoexcitation of the incipient ferroelectric
potassium-tantalate, \kto. %
We use ultrafast near-UV (central wavelength 266\,nm and 50 fs pulse
duration) laser light to photoexcite charge carriers across the gap and probe
the ultrafast lattice dynamics by recording the x-ray diffuse intensity
throughout multiple Brillouin zones using pulses from the Linac Coherent Light
Source (LCLS) (central wavelength 1.3\,\AA\, and ~fs pulse duration). We
observe changes in the diffuse intensity that we conclude are associated with a
hardening of the soft transverse optical and transverse acoustic phonon
branches along to and to . Using ground- and
excited-state interatomic force constants from density functional theory (DFT)
and assuming the phonon populations can be described by a time-dependent
temperature, we fit the quasi-equilibrium thermal diffuse intensity to the
experimental time-dependent intensity. We obtain the instantaneous lattice
temperature and density of photoexcited charge carriers as a function of time
delay. The DFT calculations demonstrate that photoexcitation transfers charge
from oxygen derived -bonding orbitals to Ta derived antibonding
orbitals, further suppressing the ferroelectric instability and increasing the
stability of the cubic, paraelectric structure.Comment: 8 pages, 4 figure