scholarly journals Systemic consequences of disorder in magnetically self-organized topological MnBi2Te4/(Bi2Te3)n superlattices

2D Materials ◽  
2021 ◽  
Author(s):  
Joanna Sitnicka ◽  
Kyungwha Park ◽  
Paweł Skupiński ◽  
Krzysztof Grasza ◽  
Anna Reszka ◽  
...  
Keyword(s):  
Density Functional ◽  
Photoemission Spectroscopy ◽  
Electron Wave ◽  
Optical Resonance ◽  
Surface Band ◽  
Functional Theory ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Self Organized ◽  
Mn Doped ◽  
The Ideal

Abstract MnBi2Te4/(Bi2Te3)n materials system has recently generated strong interest as a natural platform for realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic MnBi2Te4/(Bi2Te3)n it is found that migration of Mn between MnBi2Te4 septuple layers (SLs) and otherwise non-magnetic Bi2Te3 quintuple layers (QLs) has systemic consequences - it induces ferromagnetic coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n ≳ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wave functions and a change of the surface band structure as compared to the ideal MnBi2Te4/(Bi2Te3)n. These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets.

scholarly journals Observation of a singular Weyl point surrounded by charged nodal walls in PtGa

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
J.-Z. Ma ◽  
Q.-S. Wu ◽  
M. Song ◽  
S.-N. Zhang ◽  
E. B. Guedes ◽  
...  
Keyword(s):  
Density Functional ◽  
Density Functional Theory Calculations ◽  
Photoemission Spectroscopy ◽  
Functional Theory ◽  
Fermi Arc ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Fermi Arcs ◽  
Surface Projection ◽  
Applicable Range ◽  
Opposite Chirality

AbstractConstrained by the Nielsen-Ninomiya no-go theorem, in all so-far experimentally determined Weyl semimetals (WSMs) the Weyl points (WPs) always appear in pairs in the momentum space with no exception. As a consequence, Fermi arcs occur on surfaces which connect the projections of the WPs with opposite chiral charges. However, this situation can be circumvented in the case of unpaired WP, without relevant surface Fermi arc connecting its surface projection, appearing singularly, while its Berry curvature field is absorbed by nontrivial charged nodal walls. Here, combining angle-resolved photoemission spectroscopy with density functional theory calculations, we show experimentally that a singular Weyl point emerges in PtGa at the center of the Brillouin zone (BZ), which is surrounded by closed Weyl nodal walls located at the BZ boundaries and there is no Fermi arc connecting its surface projection. Our results reveal that nontrivial band crossings of different dimensionalities can emerge concomitantly in condensed matter, while their coexistence ensures the net topological charge of different dimensional topological objects to be zero. Our observation extends the applicable range of the original Nielsen-Ninomiya no-go theorem which was derived from zero dimensional paired WPs with opposite chirality.

scholarly journals Acceleration of hydrogen absorption by palladium through surface alloying with gold

2018 ◽  
Vol 115 (31) ◽  
pp. 7896-7900 ◽  
Author(s):  
Kazuhiro Namba ◽  
Shohei Ogura ◽  
Satoshi Ohno ◽  
Wen Di ◽  
Koichi Kato ◽  
...  
Keyword(s):  
Hydrogen Absorption ◽  
Density Functional ◽  
Depth Profiling ◽  
Thermal Desorption Spectroscopy ◽  
Photoemission Spectroscopy ◽  
Nuclear Reaction Analysis ◽  
Functional Theory ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Surface Hydrogen ◽  
Hydrogen Penetration

Enhancement of hydrogen (H) absorption kinetics improves the performance of hydrogen-purifying membranes and hydrogen-storage materials, which is necessary for utilizing hydrogen as a carbon-free energy carrier. Pd–Au alloys are known to show higher hydrogen solubility than pure Pd. However, the effect of Au on the hydrogen penetration from the surface into the subsurface region has not been clarified so far. Here, we investigate the hydrogen absorption at Pd–Au surface alloys on Pd(110) by means of thermal desorption spectroscopy (TDS) and hydrogen depth profiling with nuclear reaction analysis (NRA). We demonstrate that alloying the Pd(110) surface with submonolayer amounts of Au dramatically accelerates the hydrogen absorption. The degree of acceleration shows a volcano-shaped form against Au coverage. This kinetic enhancement is explained by a reduced penetration barrier mainly caused by a destabilization of chemisorbed surface hydrogen, which is supported by density-functional-theory (DFT) calculations. The destabilization of chemisorbed surface hydrogen is attributed to the change of the surface electronic states as observed by angle-resolved photoemission spectroscopy (ARPES). If generalized, these discoveries may lead to improving and controlling the hydrogen transport across the surfaces of hydrogen-absorbing materials.

scholarly journals Sixfold fermion near the Fermi level in cubic PtBi2

2021 ◽  
Vol 10 (1) ◽  
Author(s):  
Setti Thirupathaiah ◽  
Y. S. Kushnirenk ◽  
Klaus Koepernik ◽  
B. R. Piening ◽  
Bernd Buechner ◽  
...  
Keyword(s):  
Fermi Level ◽  
Density Functional ◽  
Photoemission Spectroscopy ◽  
Functional Theory ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Ideal Candidate ◽  
Close Proximity ◽  
Weyl Semimetals ◽  
Crossing Point ◽  
Good Agreement

We show that the cubic compound PbBi2 is a topological semimetal hosting a sixfold band touching point in close proximity to the Fermi level. Using angle-resolved photoemission spectroscopy, we map the band structure of the system, which is in good agreement with results from density functional theory. Further, by employing a low energy effective Hamiltonian valid close to the crossing point, we study the effect of a magnetic field on the sixfold fermion. The latter splits into a total of twenty Weyl cones for a Zeeman field oriented in the diagonal, (111) direction. Our results mark cubic PbBi2 as an ideal candidate to study the transport properties of gapless topological systems beyond Dirac and Weyl semimetals.

scholarly journals Flat epitaxial quasi-1D phosphorene chains

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Wei Zhang ◽  
Hanna Enriquez ◽  
Yongfeng Tong ◽  
Andrew J. Mayne ◽  
Azzedine Bendounan ◽  
...  
Keyword(s):  
Density Functional ◽  
Density Functional Theory Calculations ◽  
Photoemission Spectroscopy ◽  
Scanning Tunneling ◽  
Tunneling Microscopy ◽  
Functional Theory ◽  
Precise Control ◽  
Angle Resolved Photoemission Spectroscopy ◽  
Room Temperature Magnetism ◽  
Topological Phase Transitions

AbstractThe emergence of peculiar phenomena in 1D phosphorene chains (P chains) has been proposed in theoretical studies, notably the Stark and Seebeck effects, room temperature magnetism, and topological phase transitions. Attempts so far to fabricate P chains, using the top-down approach starting from a few layers of bulk black phosphorus, have failed to produce reliably precise control of P chains. We show that molecular beam epitaxy gives a controllable bottom-up approach to grow atomically thin, crystalline 1D flat P chains on a Ag(111) substrate. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations reveal that the armchair-shaped chains are semiconducting with an intrinsic 1.80 ± 0.20 eV band gap. This could make these P chains an ideal material for opto-electronic devices.

scholarly journals Tuning Bandgaps of Mixed Halide and Oxide Perovskites CsSnX3 (X=Cl, I), and SrBO3 (B=Rh, Ti)

2021 ◽  
Vol 11 (15) ◽  
pp. 6862
Author(s):  
Hongzhe Wen ◽  
Xuan Luo
Keyword(s):  
Solar Energy ◽  
Electronic Properties ◽  
Density Functional ◽  
Photovoltaic Cell ◽  
Photovoltaic Properties ◽  
Functional Theory ◽  
New Approach ◽  
Energy Applications ◽  
Valence Bands ◽  
The Ideal

Perovskites have recently attracted interest in the field of solar energy due to their excellent photovoltaic properties. We herein present a new approach to the composition of lead free perovskites via mixing of halide and oxide perovskites that share the cubic ABX3 structure. Using first-principles calculations through Density Functional Theory, we systematically investigated the atomic and electronic structures of mixed perovskite compounds composed of four cubic ABX3 perovskites. Our result shows that the B and X atoms play important roles in their band structure. On the other hand, their valence bands contributed by O-2p, Rh-4p, and Ti-3p orbitals, and their electronic properties were determined by Rh-O and Ti-O bonds. With new understandings of the electronic properties of cubic halide or oxide perovskites, we lastly combined the cubic perovskites in various configurations to improve stability and tune the bandgap to values desirable for photovoltaic cell applications. Our investigations suggest that the mixed perovskite compound Cs2Sn2Cl3I3Sr2TiRhO6 produced a bandgap of 1.2 eV, which falls into the ideal range of 1.0 to 1.7 eV, indicating high photo-conversion efficiency and showing promise towards solar energy applications.

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