Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological ...superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe
Se
(
= 0.45; superconducting transition temperature
= 14.5 kelvin) hosts Dirac-cone-type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below
Our study shows that the surface states of FeTe
Se
are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.
Control of the phase transition from topological to normal insulators can allow for an on/off switching of spin current. While topological phase transitions have been realized by elemental ...substitution in semiconducting alloys, such an approach requires preparation of materials with various compositions. Thus it is quite far from a feasible device application, which demands a reversible operation. Here we use angle-resolved photoemission spectroscopy and spin- and angle-resolved photoemission spectroscopy to visualize the strain-driven band-structure evolution of the quasi-one-dimensional superconductor TaSe3. We demonstrate that it undergoes reversible strain-induced topological phase transitions from a strong topological insulator phase with spin-polarized, quasi-one-dimensional topological surface states, to topologically trivial semimetal and band insulating phases. The quasi-one-dimensional superconductor TaSe3 provides a suitable platform for engineering the topological spintronics, for example as an on/off switch for a spin current that is robust against impurity scattering.Angle-resolved photoemission spectroscopy is used to track the evolution of the electronic band structure of TaSe3 across a strain-driven topological phase transition.
Abstract
A quantum spin Hall (QSH) insulator hosts topological states at the one-dimensional (1D) edge, along which backscattering by nonmagnetic impurities is strictly prohibited. Its 3D analogue, a ...weak topological insulator (WTI), possesses similar quasi-1D topological states confined at side surfaces. The enhanced confinement could provide a route for dissipationless current and better advantages for applications relative to strong topological insulators (STIs). However, the topological side surface is usually not cleavable and is thus hard to observe. Here, we visualize the topological states of the WTI candidate ZrTe
5
by spin and angle-resolved photoemission spectroscopy (ARPES): a quasi-1D band with spin-momentum locking was revealed on the side surface. We further demonstrate that the bulk band gap is controlled by external strain, realizing a more stable WTI state or an ideal Dirac semimetal (DS) state. The highly directional spin-current and the tunable band gap in ZrTe
5
will provide an excellent platform for applications.
Spin-momentum locking is essential to the spin-split Fermi surfaces of inversion-symmetry broken materials, which are caused by either Rashba-type or Zeeman-type spin-orbit coupling (SOC). While the ...effect of Zeeman-type SOC on superconductivity has experimentally been shown recently, that of Rashba-type SOC remains elusive. Here we report on convincing evidence for the critical role of the spin-momentum locking on crystalline atomic-layer superconductors on surfaces, for which the presence of the Rashba-type SOC is demonstrated. In-situ electron transport measurements reveal that in-plane upper critical magnetic field is anomalously enhanced, reaching approximately three times the Pauli limit at T = 0. Our quantitative analysis clarifies that dynamic spin-momentum locking, a mechanism where spin is forced to flip at every elastic electron scattering, suppresses the Cooper pair-breaking parameter by orders of magnitude and thereby protects superconductivity. The present result provides a new insight into how superconductivity can survive the detrimental effects of strong magnetic fields and exchange interactions.
We report on a photoemission microscopy apparatus using a 10.9-eV laser developed at the National Institute for Materials Science (NIMS). Our spectrometer realizes photoemission spectroscopy with a ...high spatial resolution by combining an imaging double energy analyzer with the electronic lens system of photoelectron emission microscopy. Energy-filtered photoelectron imaging is available in both real and momentum spaces. The spatial resolution in the real space mode is ∼30 nm. We show energy-filtered photoelectron images of a silver grid-patterned sample in real space and the band mapping of Au(111) in momentum space to demonstrate the performance of our spectrometer.
Low-dimensional van der Waals materials have been extensively studied as a platform with which to generate quantum effects. Advancing this research, topological quantum materials with van der Waals ...structures are currently receiving a great deal of attention. Here, we use the concept of designing topological materials by the van der Waals stacking of quantum spin Hall insulators. Most interestingly, we find that a slight shift of inversion centre in the unit cell caused by a modification of stacking induces a transition from a trivial insulator to a higher-order topological insulator. Based on this, we present angle-resolved photoemission spectroscopy results showing that the real three-dimensional material Bi
Br
is a higher-order topological insulator. Our demonstration that various topological states can be selected by stacking chains differently, combined with the advantages of van der Waals materials, offers a playground for engineering topologically non-trivial edge states towards future spintronics applications.
We have developed a novel photoemission microscopy apparatus employing a vacuum ultraviolet laser. This setup combines photoemission electron microscopy (PEEM) with a time-of-flight detector, ...facilitating rapid visualization of electronic states in both real and momentum space. Achieving a spatial resolution of 70 nm, attributed to the PEEM lens system, we showcase the full band mapping of a Bi(111) single crystal film using angle-resolved photoemission spectroscopy within a short acquisition time.
ABSTRACTHarnessing electron spin is crucial in developing energy-saving and high-speed devices for the next generation. In this scheme, visualizing spin-polarized electronic states aids in designing ...and developing new materials and devices. Spin-resolved photoemission spectroscopy provides information on the spin-polarized electronic states. To investigate the spin-polarized electronic states in microscopic materials and devices, spin-resolved photoemission spectroscopy requires spatial resolution in a sub-micrometer scale. Here we show the imaging-type spin-resolved photoemission microscopy (iSPEM) with an ultraviolet laser developed at the National Institutes for Materials Science (NIMS). Our iSPEM achieves a spatial resolution of 420 nm, drastically improving by more than an order of magnitude compared to conventional spin-resolved photoemission spectroscopy instruments. Besides, the multichannel spin detector significantly reduces the data acquisition time by four orders of magnitude compared to the conventional instruments. The iSPEM machine elucidates the spin-polarized electronic states of sub-micrometer scale materials, polycrystals, device structure samples, and so on, which have yet to be the target of conventional spin-resolved photoemission spectroscopy.
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