28 research outputs found
Interplay between ferromagnetism, surface states, and quantum corrections in a magnetically doped topological insulator
The breaking of time-reversal symmetry by ferromagnetism is predicted to
yield profound changes to the electronic surface states of a topological
insulator. Here, we report on a concerted set of structural, magnetic,
electrical and spectroscopic measurements of \MBS thin films wherein
photoemission and x-ray magnetic circular dichroism studies have recently shown
surface ferromagnetism in the temperature range 15 K K,
accompanied by a suppressed density of surface states at the Dirac point.
Secondary ion mass spectroscopy and scanning tunneling microscopy reveal an
inhomogeneous distribution of Mn atoms, with a tendency to segregate towards
the sample surface. Magnetometry and anisotropic magnetoresistance measurements
are insensitive to the high temperature ferromagnetism seen in surface studies,
revealing instead a low temperature ferromagnetic phase at K.
The absence of both a magneto-optical Kerr effect and anomalous Hall effect
suggests that this low temperature ferromagnetism is unlikely to be a
homogeneous bulk phase but likely originates in nanoscale near-surface regions
of the bulk where magnetic atoms segregate during sample growth. Although the
samples are not ideal, with both bulk and surface contributions to electron
transport, we measure a magnetoconductance whose behavior is qualitatively
consistent with predictions that the opening of a gap in the Dirac spectrum
drives quantum corrections to the conductance in topological insulators from
the symplectic to the orthogonal class.Comment: To appear in Phys. Rev.
Observation of Quantum-Tunneling Modulated Spin Texture in Ultrathin Topological Insulator Bi2Se3 Films
Understanding the spin-texture behavior of boundary modes in ultrathin
topological insulator films is critically essential for the design and
fabrication of functional nano-devices. Here by using spin-resolved
photoemission spectroscopy with p-polarized light in topological insulator
Bi2Se3 thin films, we report tunneling-dependent evolution of spin
configuration in topological insulator thin films across the metal-to-insulator
transition. We observe strongly binding energy- and wavevector-dependent spin
polarization for the topological surface electrons in the ultra-thin
gapped-Dirac-cone limit. The polarization decreases significantly with enhanced
tunneling realized systematically in thin insulating films, whereas magnitude
of the polarization saturates to the bulk limit faster at larger wavevectors in
thicker metallic films. We present a theoretical model which captures this
delicate relationship between quantum tunneling and Fermi surface spin
polarization. Our high-resolution spin-based spectroscopic results suggest that
the polarization current can be tuned to zero in thin insulating films forming
the basis for a future spin-switch nano-device.Comment: To appear in Nature Communications (2014); Expanded version of
http://arxiv.org/abs/1307.548
Strain Engineering a Charge Density Wave Phase in Transition Metal Dichalcogenide 1T-VSe
We report a rectangular charge density wave (CDW) phase in strained
1T-VSe thin films synthesized by molecular beam epitaxy on c-sapphire
substrates. The observed CDW structure exhibits an unconventional rectangular
4a{\times}{\sqrt{3a}} periodicity, as opposed to the previously reported
hexagonal structure in bulk crystals and exfoliated thin layered
samples. Tunneling spectroscopy shows a strong modulation of the local density
of states of the same CDW periodicity and an energy gap of
meV. The CDW energy gap evolves into a full gap at
temperatures below 500 mK, indicating a transition to an insulating phase at
ultra-low temperatures. First-principles calculations confirm the stability of
both and structures arising from soft modes in
the phonon dispersion. The unconventional structure becomes preferred in the
presence of strain, in agreement with experimental findings
Hedgehog Spin-texture and Berry's Phase tuning in a Magnetic Topological Insulator
Understanding and control of spin degrees of freedom on the surfaces of
topological materials are key to future applications as well as for realizing
novel physics such as the axion electrodynamics associated with time-reversal
(TR) symmetry breaking on the surface. We experimentally demonstrate
magnetically induced spin reorientation phenomena simultaneous with a
Dirac-metal to gapped-insulator transition on the surfaces of manganese-doped
Bi2Se3 thin films. The resulting electronic groundstate exhibits unique
hedgehog-like spin textures at low energies, which directly demonstrate the
mechanics of TR symmetry breaking on the surface. We further show that an
insulating gap induced by quantum tunnelling between surfaces exhibits spin
texture modulation at low energies but respects TR invariance. These spin
phenomena and the control of their Fermi surface geometrical phase first
demonstrated in our experiments pave the way for the future realization of many
predicted exotic magnetic phenomena of topological origin.Comment: 38 pages, 18 Figures, Includes new text, additional datasets and
interpretation beyond arXiv:1206.2090, for the final published version see
Nature Physics (2012