69 research outputs found
Large gap quantum spin Hall insulator, massless Dirac fermions and bilayer graphene analogue in InAs/Ga(In)Sb heterostructures
The quantum spin Hall insulator (QSHI) state has been demonstrated in two
semiconductor systems - HgTe/CdTe quantum wells (QWs) and InAs/GaSb QW
bilayers. Unlike the HgTe/CdTe QWs, the inverted band gap in InAs/GaSb QW
bilayers does not open at the point of the Brillouin zone, preventing
the realization of massless Dirac fermions. Here, we propose a new class of
semiconductor systems based on InAs/Ga(In)Sb multilayers, hosting a QSHI state,
a graphene-like phase and a bilayer graphene analogue, depending on their layer
thicknesses and geometry. The QSHI gap in the novel structures can reach up to
60 meV for realistic design and parameters. This value is twice as high as the
thermal energy at room temperature and significantly extends the application
potential of III-V semiconductor-based topological devices.Comment: 5 pages, 4 figure
Phase transitions in two tunnel-coupled HgTe quantum wells. Bilayer graphene analogy and beyond
HgTe quantum wells possess remarkable physical properties as for instance the
quantum spin Hall state and the 'single-valley' analog of graphene, depending
on their layer thicknesses and barrier composition. However, double HgTe
quantum wells yet contain more fascinating and still unrevealed features. Here
we report on the study of the quantum phase transitions in tunnel-coupled HgTe
layers separated by CdTe barrier. We demonstrate that this system has a 3/2
pseudo spin degree of freedom, which features a number of particular properties
associated with the spin-dependent coupling between HgTe layers. We discover a
specific metal phase arising in a wide range of HgTe and CdTe layer
thicknesses, in which a gapless bulk and a pair of helical edge states coexist.
This phase holds some properties of bilayer graphene such as an unconventional
quantum Hall effect and an electrically-tunable band gap. In this 'bilayer
graphene' phase, electric field opens the band gap and drives the system into
the quantum spin Hall state. Furthermore, we discover a new type of quantum
phase transition arising from a mutual inversion between second electron- and
hole-like subbands. This work paves the way towards novel materials based on
multi-layered topological insulators
Spin splitting of surface states in HgTe quantum wells
We report on beating appearance in Shubnikov-de Haas oscillations in
conduction band of 18-22nm HgTe quantum wells under applied top-gate voltage.
Analysis of the beatings reveals two electron concentrations at the Fermi level
arising due to Rashba-like spin splitting of the first conduction subband H1.
The difference dN_s in two concentrations as a function of the gate voltage is
qualitatively explained by a proposed toy electrostatic model involving the
surface states localized at quantum well interfaces. Experimental values of
dN_s are also in a good quantitative agreement with self-consistent
calculations of Poisson and Schrodinger equations with eight-band kp
Hamiltonian. Our results clearly demonstrate that the large spin splitting of
the first conduction subband is caused by surface nature of states
hybridized with the heavy-hole band.Comment: 7 pages, 7 figure
Pressure and temperature driven phase transitions in HgTe quantum wells
We present theoretical investigations of pressure and temperature driven
phase transitions in HgTe quantum wells grown on CdTe buffer. Using the 8-band
\textbf{kp} Hamiltonian we calculate evolution of energy band structure
at different quantum well width with hydrostatic pressure up to 20 kBar and
temperature ranging up 300 K. In particular, we show that in addition to
temperature, tuning of hydrostatic pressure allows to drive transitions between
semimetal, band insulator and topological insulator phases. Our realistic band
structure calculations reveal that the band inversion under hydrostatic
pressure and temperature may be accompanied by non-local overlapping between
conduction and valence bands. The pressure and temperature phase diagrams are
presented.Comment: 9 pages, 8 figures + Supplemental material (5 pages
Temperature-induced topological phase transition in HgTe quantum wells
We report a direct observation of temperature-induced topological phase
transition between trivial and topological insulator in HgTe quantum well. By
using a gated Hall bar device, we measure and represent Landau levels in fan
charts at different temperatures and we follow the temperature evolution of a
peculiar pair of "zero-mode" Landau levels, which split from the edge of
electron-like and hole-like subbands. Their crossing at critical magnetic field
is a characteristic of inverted band structure in the quantum well. By
measuring the temperature dependence of , we directly extract the critical
temperature , at which the bulk band-gap vanishes and the topological
phase transition occurs. Above this critical temperature, the opening of a
trivial gap is clearly observed.Comment: 5 pages + Supplemental Materials; Phys. Rev. Lett. (accepted
Temperature-driven single-valley Dirac fermions in HgTe quantum wells
We report on temperature-dependent magnetospectroscopy of two HgTe/CdHgTe
quantum wells below and above the critical well thickness . Our results,
obtained in magnetic fields up to 16 T and temperature range from 2 K to 150 K,
clearly indicate a change of the band-gap energy with temperature. The quantum
well wider than evidences a temperature-driven transition from
topological insulator to semiconductor phases. At the critical temperature of
90 K, the merging of inter- and intra-band transitions in weak magnetic fields
clearly specifies the formation of gapless state, revealing the appearance of
single-valley massless Dirac fermions with velocity of
ms. For both quantum wells, the energies extracted from
experimental data are in good agreement with calculations on the basis of the
8-band Kane Hamiltonian with temperature-dependent parameters.Comment: 5 pages, 3 figures and Supplemental Materials (4 pages
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