31 research outputs found
Work function of bulk-insulating topological insulator Bi2-xSbxTe3-ySey
Recent discovery of bulk insulating topological insulator (TI)
Bi2-xSbxTe3-ySey paved a pathway toward practical device application of TIs.
For realizing TI-based devices, it is necessary to contact TIs with a metal.
Since the band-bending at the interface dominates the character of devices,
knowledge of TIs' work function is of essential importance. We have determined
the compositional dependence of work function in Bi2-xSbxTe3-ySey by
high-resolution photoemission spectroscopy. The obtained work-function values
(4.95-5.20 eV) show a systematic variation with the composition, well tracking
the energy shift of the surface chemical potential seen by angle-resolved
photoemission spectroscopy. The present result serves as a useful guide for
developing TI-based electronic devices.Comment: 4pages, 2 figure
Direct Evidence for the Dirac-Cone Topological Surface States in Ternary Chalcogenide TlBiSe2
We have performed high-resolution angle-resolved photoemission spectroscopy
on TlBiSe2, which is a member of the ternary chalcogenides theoretically
proposed as candidates for a new class of three-dimensional topological
insulators. By measuring the energy band dispersions over the entire surface
Brillouin zone, we found a direct evidence for a non-trivial surface metallic
state showing a X-shaped energy dispersion within the bulk band gap. The
present result unambiguously establishes that TlBiSe2 is a strong topological
insulator with a single Dirac cone at the Brillouin-zone center. The observed
bulk band gap of 0.4 eV is the largest among known topological insulators,
making TlBiSe2 the most promising material for studying room-temperature
topological phenomena.Comment: 4 pages, 4 figure
Observation of band crossings protected by nonsymmorphic symmetry in the layered ternary telluride Ta3SiTe6
We have performed angle-resolved photoemission spectroscopy of layered
ternary telluride Ta3SiTe6 which is predicted to host nodal lines associated
with nonsymmorphic crystal symmetry. We found that the energy bands in the
valence-band region show Dirac-like dispersions which present a band degeneracy
at the R point of the bulk orthorhombic Brillouin zone. This band degeneracy
extends one-dimensionally along the whole SR high-symmetry line, forming the
nodal lines protected by the glide mirror symmetry of the crystal. We also
observed a small band splitting near EF which supports the existence of
hourglass-type dispersions predicted by the calculation. The present results
provide an excellent opportunity to investigate the interplay between exotic
nodal fermions and nonsymmorphic crystal symmetry.Comment: 6 pages, 4 figure
Observation of a Dirac nodal line in AlB2
We have performed angle-resolved photoemission spectroscopy of AlB2 which is
isostructural to high-temperature superconductor MgB2. Using soft-x-ray
photons, we accurately determined the three-dimensional bulk band structure and
found a highly anisotropic Dirac-cone band at the K point in the bulk hexagonal
Brillouin zone. This band disperses downward on approaching the H point while
keeping its degeneracy at the Dirac point, producing a characteristic Dirac
nodal line along the KH line. We also found that the band structure of AlB2 is
regarded as a heavily electron-doped version of MgB2 and is therefore well
suited for fully visualizing the predicted Dirac nodal line. The present
results suggest that (Al,Mg)B2 system is a promising platform for studying the
interplay among Dirac nodal line, carrier doping, and possible topological
superconducting properties.Comment: 6 pages, 3 figure
Charge order with unusual star-of-David lattice in monolayer NbTe2
Interplay between fermiology and electron correlation is crucial for
realizing exotic quantum phases. Transition-metal dichalcogenide (TMD) 1T-TaS2
has sparked a tremendous attention owing to its unique Mott-insulating phase
coexisting with the charge-density wave (CDW). However, how the fermiology and
electron correlation are associated with such properties has yet to be claried.
Here we demonstrate that monolayer 1T-NbTe2 is a new class of two-dimensional
TMD which has the star-of-David lattice similarly to bulk TaS2 and
isostructural monolayer NbSe2, but exhibits a metallic ground state with an
unusual lattice periodicity root19xroot19 characterized by the sparsely
occupied star-of-David lattice. By using angle-resolved photoemission and
scanning-tunneling spectroscopies in combination with first-principles
band-structure calculations, we found that the hidden Fermi-surface nesting and
associated CDW formation are a primary cause to realize this unique correlated
metallic state with no signature of Mott gap. The present result points to a
vital role of underlying fermiology to characterize the Mott phase of TMDs.Comment: To be published in Physical Review
Unusual surface states associated with the PT-symmetry breaking and antiferromagnetic band folding in NdSb
We have performed micro-focused angle-resolved photoemission spectroscopy on
NdSb which exhibits the type-I antiferromagnetism below TN = 16 K. We succeeded
in selectively observing the band structure for all the three types of single-q
antiferromagnetic (AF) domains at the surface. We found that the two of three
surfaces whose AF-ordering vector lies within the surface plane commonly show
two-fold-symmetric surface states (SSs) around the bulk-band edges, whereas the
other surface with an out-of-plane AF-ordering vector displays
four-fold-symmetric shallow electronlike SS at the Brillouin-zone center. We
suggest that these SSs commonly originate from the combination of the PT
(space-inversion and time-reversal) symmetry breaking at the surface and the
band folding due to the AF order. The present results pave a pathway toward
understanding the relationship between the symmetry and the surface electronic
states in antiferromagnets.Comment: 20 pages, 5 figure
Surface-termination-dependent electronic states in kagome superconductors AV3Sb5 (A = K, Rb, Cs) studied by micro-ARPES
Recently discovered kagome superconductors AV3Sb5 (A = K, Rb, Cs) exhibit
exotic bulk and surface physical properties such as charge-density wave (CDW)
and chirality, whereas their origins remain unresolved. By using micro-focused
angle-resolved photoemission spectroscopy, we discovered that AV3Sb5 commonly
exhibits two distinct polar surfaces depending on the termination; electron-
and hole-doped ones for the A- and Sb-termination, respectively. We observed
that the kagome-derived band shows a clear splitting in the A-terminated
surface while it is absent in the Sb-terminated counterpart, indicative of the
polarity-dependent CDW at the surface. Close comparison of the band-dependent
splitting reveals that the three-dimensional CDW structure of the K-terminated
surface is different from that of the Rb- or Cs-terminated surface, suggesting
the diversity of the CDW ground state. These results provide important insight
into the origin of CDW in kagome superconductors AV3Sb5.Comment: 10 pages, 8 figure
Unusual change in the Dirac-cone energy band upon a two-step magnetic transition in CeBi
We have performed angle-resolved photoemission spectroscopy (ARPES) on CeBi
which undergoes a two-step antiferromagnetic (AF) transition with temperature.
Soft-x-ray ARPES has revealed the inverted band structure at the X point of
bulk Brillouin zone for CeBi (and also for LaBi) as opposed to LaSb with
non-inverted band structure. Low-energy ARPES on CeBi has revealed the
Dirac-cone band at the Gamma point in the paramagnetic phase associated with
the bulk band inversion. On the other hand, a double Dirac-cone band appears on
entering the first AF phase at T = 25 K, whereas a single Dirac-cone band
recovers below the second AF transition at T = 14 K. The present result
suggests an intricate interplay between antiferromagnetism and topological
surface states in CeBi.Comment: 8 pages, 6 figure