19 research outputs found
A combined laser-based ARPES and 2PPES study of Td-WTe
Laser-based angle-resolved photoemission spectroscopy (ARPES) and two-photon
photoemission spectroscopy (2PPES) are employed to study the valence electronic
structure of the Weyl semimetal candidate Td-WTe along two high symmetry
directions and for binding energies between -1 eV and 5 eV. The
experimental data show a good agreement with band structure calculations.
Polarization dependent measurements provide furthermore information on initial
and intermediate state symmetry properties with respect to the mirror plane of
the Td structure of WTe
Visualization of the electronic phase separation in superconducting K x Fe 2-y Se 2
AbstractType-II iron-based superconductors (Fe-SCs), the alkali-metal-intercalated iron selenide AxFe2−ySe2 (A = K, Tl, Rb, etc.) with a superconducting transition temperature of 32 K, exhibit unique properties such as high Néel temperature, Fe-vacancies ordering, antiferromagnetically ordered insulating state in the phase diagram, and mesoscopic phase separation in the superconducting materials. In particular, the electronic and structural phase separation in these systems has attracted intensive attention since it provides a platform to unveil the insulating parent phase of type-II Fe-SCs that mimics the Mott parent phase in cuprates. In this work, we use spatial- and angle-resolved photoemission spectroscopy to study the electronic structure of superconducting KxFe2−ySe2. We observe clear electronic phase separation of KxFe2−ySe2 into metallic islands and insulating matrix, showing different K and Fe concentrations. While the metallic islands show strongly dispersive bands near the Fermi level, the insulating phase shows an energy gap up to 700 meV and a nearly flat band around 700 meV below the Fermi energy, consistent with previous experimental and theoretical results on the superconducting K1−xFe2Se2 (122 phase) and Fe-vacancy ordered K0.8Fe1.6Se2 (245 phase), respectively. Our results not only provide important insights into the mysterious composition of phase-separated superconducting and insulating phases of KxFe2−ySe2, but also present their intrinsic electronic structures, which will shed light on the comprehension of the unique physics in type-II Fe-SCs
Towards Layer-Selective Quantum Spin Hall Channels in Weak Topological Insulator Bi4Br2I2
Weak topological insulators, constructed by stacking quantum spin Hall
insulators with weak interlayer coupling, offer promising quantum electronic
applications through topologically nontrivial edge channels. However, the
currently available weak topological insulators are stacks of the same quantum
spin Hall layer with translational symmetry in the out-of-plane direction,
leading to the absence of the channel degree of freedom for edge states. Here,
we study a candidate weak topological insulator, Bi4Br2I2, which is alternately
stacked by three different quantum spin Hall insulators, each with tunable
topologically non-trivial edge states. Our angle-resolved photoemission
spectroscopy and first-principles calculations show that an energy gap opens at
the crossing points of different Dirac cones correlated with different layers
due to the interlayer interaction. This is essential to achieve the tunability
of topological edge states as controlled by varying the chemical potential. Our
work offers a perspective for the construction of tunable quantized conductance
devices for future spintronic applications
Topology hierarchy of transition metal dichalcogenides built from quantum spin Hall layers
The evolution of the physical properties of two-dimensional material from
monolayer limit to the bulk reveals unique consequences from dimension
confinement and provides a distinct tuning knob for applications. Monolayer
1T'-phase transition metal dichalcogenides (1T'-TMDs) with ubiquitous quantum
spin Hall (QSH) states are ideal two-dimensional building blocks of various
three-dimensional topological phases. However, the stacking geometry was
previously limited to the bulk 1T'-WTe2 type. Here, we introduce the novel
2M-TMDs consisting of translationally stacked 1T'-monolayers as promising
material platforms with tunable inverted bandgaps and interlayer coupling. By
performing advanced polarization-dependent angle-resolved photoemission
spectroscopy as well as first-principles calculations on the electronic
structure of 2M-TMDs, we revealed a topology hierarchy: 2M-WSe2, MoS2, and
MoSe2 are weak topological insulators (WTIs), whereas 2M-WS2 is a strong
topological insulator (STI). Further demonstration of topological phase
transitions by tunning interlayer distance indicates that band inversion
amplitude and interlayer coupling jointly determine different topological
states in 2M-TMDs. We propose that 2M-TMDs are parent compounds of various
exotic phases including topological superconductors and promise great
application potentials in quantum electronics due to their flexibility in
patterning with two-dimensional materials
Superconductivity in trilayer nickelate La4Ni3O10 under pressure
Nickelates gained a great deal of attention due to their similar crystal and
electronic structures of cuprates over the past few decades. Recently,
superconductivity with transition temperature exceeding liquid-nitrogen
temperature is discovered in La3Ni2O7, which belong to the Ruddlesden-Popper
(RP) phases Lan+1NinO3n+1 with n = 2. In this work, we go further and find
pressure-induced superconductivity in another RP phase La4Ni3O10 (n = 3) single
crystals. Our angle-resolved photoemission spectroscopy (ARPES) experiment
suggest that the electronic structure of La4Ni3O10 is very similar to that of
La3Ni2O7. We find that the density-wave like anomaly in resistivity is
progressively suppressed with increasing pressure. A typical phase diagram is
obtained with the maximum Tc of 21 Kelvin. Our study sheds light on the
exploration of unconventional superconductivity in nickelates.Comment: 16 pages, 5 figure
Observation of topological electronic structure in quasi-1D superconductor TaSe3
Topological superconductors (TSCs), with the capability to host Majorana
bound states that can lead to non-Abelian statistics and application in quantum
computation, have been one of the most intensively studied topics in condensed
matter physics recently. Up to date, only a few compounds have been proposed as
candidates of intrinsic TSCs, such as doped topological insulator CuxBi2Se3 and
iron-based superconductor FeTe0.55Se0.45. Here, by carrying out synchrotron and
laser based angle-resolved photoemission spectroscopy (ARPES), we
systematically investigated the electronic structure of a quasi-1D
superconductor TaSe3, and identified the nontrivial topological surface states.
In addition, our scanning tunneling microscopy (STM) study revealed a clean
cleaved surface with a persistent superconducting gap, proving it suitable for
further investigation of potential Majorana modes. These results prove TaSe3 as
a stoichiometric TSC candidate that is stable and exfoliable, therefore a great
platform for the study of rich novel phenomena and application potentials.Comment: to appear in Matte
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Electronic origin of half-metal to semiconductor transition and colossal magnetoresistance in spinel HgCr2Se4
Half metals are ferromagnets hosting spin-polarized conducting carriers and are crucial for spintronics applications. The chromium spinel HgCr2Se4 represents a unique type of half-metal, which features a half-metal to semiconductor transition (HMST) and exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Using angle-resolved photoemission spectroscopy, we find that the Fermi surface of n-type HgCr2Se4 (n-HgCr2Se4) consists of a single electron pocket which moves above the Fermi level (EF) upon the FM-PM transition, leading to the HMST. Such a Lifshitz transition manifests a giant band splitting which originates from the exchange interaction unveiled with a specific chemical nonstoichiometry. The exchange band splitting and the chemical nonstoichiometry are two key ingredients to the HMST and CMR, consistent with our ab initio calculation. Our findings provide spectroscopic evidence of the electronic origin of the anomalous properties of HgCr2Se4, which address the unique phase transition in half-metals