11 research outputs found
Dirac parameters and topological phase diagram of Pb1-xSnxSe from magneto-spectroscopy
Pb1-xSnxSe hosts 3D massive Dirac fermions across the entire composition
range for which the crystal structure is cubic. In this work, we present a
comprehensive experimental mapping of the 3D band structure parameters of
Pb1-xSnxSe as a function of composition and temperature. We cover a parameter
space spanning the band inversion that yields its topological crystalline
insulator phase. A non-closure of the energy gap is evidenced in the vicinity
of this phase transition. Using magnetooptical Landau level spectroscopy, we
determine the energy gap, Dirac velocity, anisotropy factor and topological
character of Pb1-xSnxSe epilayers grown by molecular beam epitaxy on BaF2
(111). Our results are evidence that Pb1-xSnxSe is a model system to study
topological phases and the nature of the phase transition.Comment: Submitte
Massive and massless Dirac fermions in Pb1-xSnxTe topological crystalline insulator probed by magneto-optical absorption
Dirac fermions in condensed matter physics hold great promise for novel
fundamental physics, quantum devices and data storage applications. IV-VI
semiconductors, in the inverted regime, have been recently shown to exhibit
massless topological surface Dirac fermions protected by crystalline symmetry,
as well as massive bulk Dirac fermions. Under a strong magnetic field (B), both
surface and bulk states are quantized into Landau levels that disperse as
B^1/2, and are thus difficult to distinguish. In this work, magneto-optical
absorption is used to probe the Landau levels of high mobility Bi-doped
Pb0.54Sn0.46Te topological crystalline insulator (111)-oriented films. The high
mobility achieved in these thin film structures allows us to probe and
distinguish the Landau levels of both surface and bulk Dirac fermions and
extract valuable quantitative information about their physical properties. This
work paves the way for future magnetooptical and electronic transport
experiments aimed at manipulating the band topology of such materials.Comment: supplementary material included, to appear in Scientific Report
Tunable Dirac interface states in topological superlattices
Relativistic Dirac fermions are ubiquitous in condensed matter physics. Their
mass is proportional to the material energy gap and the ability to control and
tune the mass has become an essential tool to engineer quantum phenomena that
mimic high energy particles and provide novel device functionalities. In
topological insulator thin films, new states of matter can be generated by
hybridizing the massless Dirac states that occur at material surfaces. In this
work, we experimentally and theoretically introduce a platform where this
hybridization can be continuously tuned: the Pb1-xSnxSe topological
superlattice. In this system, topological Dirac states occur at the interfaces
between a topological crystalline insulator Pb1-xSnxSe and a trivial insulator,
realized in the form of topological quantum wells (TQW) epitaxially stacked on
top of each other. Using magnetooptical transmission spectroscopy on high
quality MBE grown Pb1-xSnxSe superlattices, we show that the penetration depth
of the TQW interface states and therefore their Dirac mass is continuously
tunable with temperature. This presents a new pathway to engineer the Dirac
mass of topological systems and paves the way towards the realization of
emergent quantum states of matter using Pb1-xSnxSe topological superlattices.Comment: See journal for supplementary material acces
Avoided level crossing at the magnetic field induced topological phase transition due to spin-orbital mixing
In 3D topological insulators, an effective closure of the bulk energy gap
with increasing magnetic field expected at a critical point can yield a band
crossing at a gapless Dirac node. Using high-field magnetooptical Landau level
spectroscopy on the topological crystalline insulator Pb1-xSnxSe, we
demonstrate that such a gap closure does not occur, and an avoided crossing is
observed as the magnetic field is swept through the critical field. We
attribute this anticrossing to orbital parity and spin mixing of the N=0
levels. Concurrently, we observe no gap closure at the topological phase
transition versus temperature suggesting that the anticrossing is a generic
property of topological phase transitions.Comment: submitte
Negative longitudinal magnetoresistance from anomalous N=0 Landau level in topological materials
Negative longitudinal magnetoresistance (NLMR) is shown to occur in
topological materials in the extreme quantum limit, when a magnetic field is
applied parallel to the excitation current. We perform pulsed and DC field
measurements on Pb1-xSnxSe epilayers where the topological state can be
chemically tuned. The NLMR is observed in the topological state, but is
suppressed and becomes positive when the system becomes trivial. In a
topological material, the lowest N=0 conduction Landau level disperses down in
energy as a function of increasing magnetic field, while the N=0 valence Landau
level disperses upwards. This anomalous behavior is shown to be responsible for
the observed NLMR. Our work provides an explanation of the outstanding question
of NLMR in topological insulators and establishes this effect as a possible
hallmark of bulk conduction in topological matter.Comment: Accepted in Physical Review Letter
Magneto-optical evidence of the topological phase transition in (111)-Pb 1-x Sn x Te
International audienc
Evidence of Fermi level pinning at the Dirac point in epitaxial multilayer graphene
International audienceWe investigate the temperature-dependent conductivity of epitaxial multilayer graphene using THz time-domain spectroscopy and find evidence that the Fermi level in quasineutral graphene layers is pinned at the Dirac point by midgap states. We demonstrate that the scattering mechanisms result from the interplay between midgap states that dominate in the vicinity of the Dirac point and short-range potentials that govern at higher energies (>8 meV). Our results highlight the potential of multilayer epitaxial graphene for probing low-energy Dirac particles and also for THz optics