Abstract
The topological materials have attracted much attention for their unique electronic structure and peculiar physical properties. ZrTe
5
has host a long-standing puzzle on its anomalous ...transport properties manifested by its unusual resistivity peak and the reversal of the charge carrier type. It is also predicted that single-layer ZrTe
5
is a two-dimensional topological insulator and there is possibly a topological phase transition in bulk ZrTe
5
. Here we report high-resolution laser-based angle-resolved photoemission measurements on the electronic structure and its detailed temperature evolution of ZrTe
5
. Our results provide direct electronic evidence on the temperature-induced Lifshitz transition, which gives a natural understanding on underlying origin of the resistivity anomaly in ZrTe
5
. In addition, we observe one-dimensional-like electronic features from the edges of the cracked ZrTe
5
samples. Our observations indicate that ZrTe
5
is a weak topological insulator and it exhibits a tendency to become a strong topological insulator when the layer distance is reduced.
Temperature dependent thermal and spectroscopic properties of Yb:LuAG crystal were presented in detail at temperature ranging from 298 to 77 K. We measured the thermal expansion coefficient, thermal ...conductivity and specific heat as functions of temperatures. Then, the thermal shock resistance parameters were evaluated, which were significantly increased from 0.75×10
6
to 10.19×10
6
W/m as temperature varies from 298 to 77 K. Moreover, the spectroscopic parameters, such as absorption and emission cross-sections and fluorescence lifetime were also determined. In order to evaluate Yb:LuAG’s potential to generate high power laser output, a numerical model was employed by finite element method and ray tracing method. We simulated the temperature distribution in Yb:LuAG crystal slab when temperature reduces from 298 to 77 K. The maximum principal stress in slab decreases from 74.8 to 29.4 MPa and the maximum principal strain decreases from 23.76×10
-5
to 7.63×10
-5
for a given heat power of 1500 W with the temperature dropping from 298 to 77 K. The simulated near-field and far-field profiles of probe beam demonstrated a better beam quality at low temperature. These results indicate that cryogenically cooled Yb:LuAG is a promising laser medium for high power output while maintaining good beam quality
The iron-based superconductors are characterized by multiple-orbital physics where all the five Fe3dorbitals get involved. The multiple-orbital nature gives rise to various novel phenomena like ...orbital-selective Mott transition, nematicity, and orbital fluctuation that provide a new route for realizing superconductivity. The complexity of multiple-orbital physics also requires us to disentangle the relationship between orbital, spin, and nematicity, and to identify dominant orbital ingredients that dictate superconductivity. The bulk FeSe superconductor provides an ideal platform to address these issues because of its simple crystal structure and unique coexistence of superconductivity and nematicity. However, the orbital nature of the low-energy electronic excitations and its relation to the superconducting gap remain controversial. Here, we report direct observation of the highly anisotropic Fermi surface and extremely anisotropic superconducting gap in the nematic state of the FeSe superconductor by high-resolution laser-based angle-resolved photoemission measurements. We find that the low-energy excitations of the entire hole pocket at the Brillouin zone center are dominated by the singledxzorbital. The superconducting gap exhibits an anticorrelation relation with thedxzspectral weight near the Fermi level; i.e., the gap size minimum (maximum) corresponds to the maximum (minimum) of thedxzspectral weight along the Fermi surface. These observations provide new insights in understanding the orbital origin of the extremely anisotropic superconducting gap in the FeSe superconductor and the relation between nematicity and superconductivity in the iron-based superconductors.
Abstract
Kagome lattices of various transition metals are versatile platforms for achieving anomalous Hall effects, unconventional charge-density wave orders and quantum spin liquid phenomena due to ...the strong correlations, spin-orbit coupling and/or magnetic interactions involved in such a lattice. Here, we use laser-based angle-resolved photoemission spectroscopy in combination with density functional theory calculations to investigate the electronic structure of the newly discovered kagome superconductor CsTi
3
Bi
5
, which is isostructural to the AV
3
Sb
5
(A = K, Rb or Cs) kagome superconductor family and possesses a two-dimensional kagome network of titanium. We directly observe a striking flat band derived from the local destructive interference of Bloch wave functions within the kagome lattice. In agreement with calculations, we identify type-II and type-III Dirac nodal lines and their momentum distribution in CsTi
3
Bi
5
from the measured electronic structures. In addition, around the Brillouin zone centre,
$${{\mathbb{Z}}}_{2}$$
Z
2
nontrivial topological surface states are also observed due to band inversion mediated by strong spin-orbit coupling.
The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and ...room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi2Se3 and Bi2Te3, upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.
The ongoing discoveries and studies of novel topological quantum materials have become an emergent and important field of condensed matter physics. Recently, Hfres ignited renewed interest as a ...candidate of a novel topological material. The single-layer Hffes is predicted to be a tWOldimensional large band gap topological insulator and can be stacked into a bulk that may host a temperatureldriven topological phase transition. Historically, Hfres attracted considerable interest for its anomalous transport properties characterized by a peculiar resistivity peak accompanied by a sign reversal carrier type. The origin of the transport anomaly remains under a hot debate. Here we report the first high-resolution laserlbased anglelresolved photoemission measurements on the temperature-dependent electronic structure in Hffes. Our results indicated that a temperature-induced Lifshitz transition occurs in Hffes, which provides a natural understanding on the origin of the transport anomaly in Hffe~. In addition, our observa- tions suggest that Hffes is a weak topological insulator that is located at the phase boundary between weak and strong topological insulators at very low temperature.
Abstract The latest discovery of high temperature superconductivity near 80 K in La 3 Ni 2 O 7 under high pressure has attracted much attention. Many proposals are put forth to understand the origin ...of superconductivity. The determination of electronic structures is a prerequisite to establish theories to understand superconductivity in nickelates but is still lacking. Here we report our direct measurement of the electronic structures of La 3 Ni 2 O 7 by high-resolution angle-resolved photoemission spectroscopy. The Fermi surface and band structures of La 3 Ni 2 O 7 are observed and compared with the band structure calculations. Strong electron correlations are revealed which are orbital- and momentum-dependent. A flat band is formed from the Ni-3d $_{{z}^{2}}$$ z 2 orbitals around the zone corner which is ~ 50 meV below the Fermi level and exhibits the strongest electron correlation. In many theoretical proposals, this band is expected to play the dominant role in generating superconductivity in La 3 Ni 2 O 7 . Our observations provide key experimental information to understand the electronic structure and origin of high temperature superconductivity in La 3 Ni 2 O 7 .
The superconducting gap symmetry is crucial in understanding the underlying superconductivity mechanism. Angle-resolved photoemission spectroscopy (ARPES) has played a key role in determining the gap ...symmetry in unconventional superconductors. However, it has been considered so far that ARPES can only measure the magnitude of the superconducting gap but not its phase; the phase has to be detected by other phase-sensitive techniques. Here we propose a method to directly detect the superconducting gap sign by ARPES. This method is successfully validated in a cuprate superconductor Bi
Sr
CaCu
O
with a well-known d-wave gap symmetry. When two bands have a strong interband interaction, the resulted electronic structures in the superconducting state are sensitive to the relative gap sign between the two bands. Our present work provides an approach to detect the gap sign and can be applied to various superconductors, particularly those with multiple orbitals like the iron-based superconductors.
Topological insulators represent a new quantum state of matter that are insulating in the bulk but metallic on the edge or surface. In the Dirac surface state, it is well-established that the ...electron spin is locked with the crystal momentum. Here we report a new phenomenon of the spin texture locking with the orbital texture in a topological insulator Bi₂Se₃. We observe light-polarization-dependent spin texture of both the upper and lower Dirac cones that constitutes strong evidence of the orbital-dependent spin texture in Bi₂Se₃. The different spin texture detected in variable polarization geometry is the manifestation of the spin-orbital texture in the initial state combined with the photoemission matrix element effects. Our observations provide a new orbital degree of freedom and a new way of light manipulation in controlling the spin structure of the topological insulators that are important for their future applications in spin-related technologies.
Three-dimensional topological insulators are characterized by insulating bulk state and metallic surface state involving relativistic Dirac fermions which are responsible for exotic quantum phenomena ...and potential applications in spintronics and quantum computations. It is essential to understand how the Dirac fermions interact with other electrons, phonons and disorders. Here we report super-high resolution angle-resolved photoemission studies on the Dirac fermion dynamics in the prototypical Bi2(Te,Se)3 topological insulators. We have directly revealed signatures of the electron-phonon coupling and found that the electron-disorder interaction dominates the scattering process. The Dirac fermion dynamics in Bi2(Te3-xSex) topological insulators can be tuned by varying the composition, x, or by controlling the charge carriers. Our findings provide crucial information in understanding and engineering the electron dynamics of the Dirac fermions for fundamental studies and potential applications.
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