Hole-like States Research Articles

Spin–orbit magneto-transport in P-type double top-gate devices

The Luttinger Hamiltonian model is employed to investigate spin–orbit magneto-transport in an indium arsenide-based p-type double top-gate device with Rashba and Zeeman effects. The analyses focus particularly on the effects of the top-gate width (L), distance between the top gates (d), potential energy amplitude (U0), and asymmetry of the top-gate width on the formation of electron-like quasi-bound states and hole-like quasi-bound states within the conduction channel. The results confirm the feasibility of controlling the transport characteristics of the charged carriers through an appropriate setting of the gate width and gate potential energy. The findings provide useful insights into the complex transport dynamics of p-type semiconductor systems and have potential implications for device engineering.

Crystal-field analysis of photoluminescence from orthorhombic Eu centers and energy transfer from host to Eu in GaN co-doped with Mg and Eu

Gallium nitride co-doped with magnesium and europium shows great potential for active layers in red light emitting diode structures due to strong and sharp luminescence emission around 620 nm. In this work, sharp and intense Eu3+ luminescence lines from the excited states of the 5DJ (J = 0, 1) multiplets to the ground states of the 7FJ (J = 0, 1, 2) multiplets have been analyzed using a C2v crystal-field equivalent operator Hamiltonian. A model of Eu centers with the C2v symmetry has been proposed to be an Eu3+ complex accompanied by either a pair of nitrogen and gallium vacancies (VN-VGa) or a pair consisting of a nitrogen vacancy and magnesium impurity (VN-MgGa) in the vicinity of the Eu ion based on the crystal-field analysis, the selection rules and the observed polarization of the Eu3+ luminescence lines. Energy transfer from the host to the Eu ions under band-to-band excitation occurs through electron-hole recombination between VN with the electron-like state and VGa or MgGa with the hole-like state; these may be associated with the shallow-trapped or deep-trapped states, respectively, proposed as the energy transfer mechanism in previous literature.

Emergence of quasi-1D spin-polarized states in ultrathin Bi films on InAs(111)A for spintronics applications.

The discovery, characterization, and control of heavy-fermion low-dimensional materials are central to nanoscience since quantum phenomena acquire an exotic and highly tunable character. In this work, through a variety of comprehensive experimental and theoretical techniques, it was observed and predicted that the synthesis of ultrathin Bi films on the InAs(111)A surface produces quasi-one-dimensional spin-polarized states, providing a platform for the realization of a unique spin-transport regime in the system. Scanning tunneling microscopy and low-energy electron diffraction measurements revealed that the InAs(111)A substrate facilitates the formation of the Bi-dimer phase of 2√3 × 3 periodicity with an admixture of the Bi-bilayer phase under submonolayer Bi deposition. X-ray photoelectron spectroscopy (XPS) measurements have shown the chemical stability of the Bi-induced phases, while spin and angle resolved photoemission spectroscopy (SARPES) observations combined with state-of-the-art DFT calculations have revealed that the electronic spectrum of the Bi-dimer phase holds a quasi-1D hole-like spin-split state at the Fermi level with advanced spin texture, whereas the Bi-bilayer phase demonstrates metallic states with large Rashba spin-splitting. The band structure of the Bi/InAs(111)A interface is discovered to hold great potential as a high-performance spintronics material fabricated in the ultimate two-dimensional limit.

Plunger gate effects on magneto transport in double-top gate spin-orbit devices

A propagation matrix method is used to investigate the quantum conductance of an n-type double-top gate device under a strong Rashba-Zeeman coupling interaction. The effects of a plunger gate potential and the total top gate length on the magneto conduction properties are investigated. Transcendental equations and analytical formulas are derived to accurately predict the resonance state and hole-like bound state structures. Furthermore, the effect of the distance of the two top gates from the plunger gate is examined for both symmetric and asymmetric structures. The analytical technique employed in this study provides an accurate and reliable approach for calculating the conductance of the quantum transport and is therefore beneficial for the design of n-type semiconductor devices.

Ballistic spin transport in DC-bias single top gate p-type narrow channel device with Zeeman–Rashba effects

The ballistic transport of holes in a p-type quasi-one-dimensional channel system is investigated without considering the coupling effect of the light and heavy holes. A top gate (TG) is used to form a potential barrier in the quantum channel with Zeeman–Rashba (ZR) effects. The Luttinger Hamiltonian and propagation matrix method are used to examine the relationship between the conductance and the incident hole energy. The results show that when the gate width L is large, an electron-like bound state (E-BS) structure is formed at the inflection point of the spin-down branch. When a positive gate potential energy is applied (U0>0), the light and heavy hole spins form an electron-like quasi-bound state (E-QBS) at the top of the branch under the hole spin. Conversely, when a negative gate potential energy (U0<0) is applied, the light and heavy hole spins form a hole-like quasi-bound state (H-QBS) structure at the bottom of the branch.

Spin quantum transport in double top-gate system

The quantum transport in an n-type two-dimensional electron gas (2DEG) with a double top gate system is investigated. A propagation matrix method is used to derive the band structure and obtain the conductance of the considered system. The results show that in the presence of the Rashba and the Zeeman interactions, resonance peaks and hole-like bound state structure are formed in the conductance spectra influenced by the gate potential and gate distance. While, under the influence of the Rashba–Dresselhaus and the Zeeman interactions, resonance peaks and inner-mode resonance conductance peaks are formed reflecting the gate potential and the gate distance. Notably, when the Dresselhaus interaction reaches a certain strength, the partial conductance of the spin-preservation internal mode becomes significant. Finally, a simple transcendental equation is found to estimate the energy of a bound state.

Quantized conductance with nonzero shot noise as a signature of Andreev edge state

Electrical conductance measurements have a limited scope in identifying Andreev edge states (AESs), which form the basis for realizing various topological excitations in quantum Hall (QH)--superconductor (SC) junctions. To unambiguously detect AESs, we measure shot noise along with electrical conductance in a graphene-based QH-SC junction at integer filling $\ensuremath{\nu}=2$. Remarkably, we find that the Fano factor of shot noise approaches half when the bias energy is less than the superconducting gap ($2\mathrm{\ensuremath{\Delta}}$), whereas it is close to zero above $2\mathrm{\ensuremath{\Delta}}$. This is striking, given that, at the same time, the electrical conductance remains quantized at $2{e}^{2}/h$ within and above $2\mathrm{\ensuremath{\Delta}}$. A quantized conductance is expected to produce zero-shot noise due to its dissipationless flow. However, at a QH-SC interface, AESs carry the current in the zero-bias limit and an equal mixing of electron- and holelike states produces half of the Poissonian shot noise with quantized conductance. The observed results are in accord with our detailed theoretical calculations of electrical conductance and shot noise based on the nonequilibrium Green's function method in the presence of disorder. Our results pave the way in using shot noise as a detection tool in the search for exotic topological excitations in QH-SC hybrids.

Quantum dots in AA-stacked bilayer graphene

While electrostatic confinement in single-layer graphene and AA-stacked bilayer graphene is precluded by Klein tunneling and the gapless energy spectrum, we theoretically show that a circular domain wall that separates domains of single-layer graphene and AA-stacked bilayer graphene can provide bound states. Solving the Dirac-Weyl equation in the presence of a global mass potential and a local electrostatic potential, we obtain the energy spectrum of these states and the corresponding radial probability densities. Depending on the mass potential profile, regular bound states can exist inside the quantum dot and topological bound states at the domain wall. Controlling the electrostatic potential inside the quantum dot enables the simultaneous presence of both types of states. We find that the number of nodes of the radial wave function of the regular bound states inside the quantum dot is equal to the radial quantum number. The energy spectra of the bound states display anticrossings, reflecting coupling of electron- and holelike states.

Asymmetric versus symmetric HgTe/CdxHg1−xTe double quantum wells: Bandgap tuning without electric field

We investigate the electron states in double asymmetric HgTe/CdxHg1−xTe quantum wells grown along the [001] direction. The subbands are computed by means of the envelope function approximation applied to the eight-band Kane k⋅p model. The asymmetry of the confining potential of the double quantum wells results in a gap opening, which is absent in the symmetric system where it can only be induced by an applied electric field. The bandgap and the subbands are affected by spin–orbit coupling, which is a consequence of the asymmetry of the confining potential. The electron-like and hole-like states are mainly confined in different quantum wells, and the enhanced hybridization between them opens a spin-dependent hybridization gap at a finite in-plane wavevector. We show that both the ratio of the widths of the two quantum wells and the mole fraction of the CdxHg1−xTe barrier control both the energy gap between the hole-like states and the hybridization gap. The energy subbands are shown to exhibit inverted ordering, and therefore, a nontrivial topological phase could emerge in the system.

Pentacene and tetracene molecules and films on H/Si(111): level alignment from hybrid density functional theory

The electronic properties of hybrid organic–inorganic semiconductor interfaces depend strongly on the alignment of the electronic carrier levels in the organic/inorganic components. In the present work, we address this energy level alignment from first principles theory for two paradigmatic organic–inorganic semiconductor interfaces, the singlet fission materials tetracene and pentacene on H/Si(111), using all-electron density functional theory calculations with a hybrid exchange–correlation functional. For isolated tetracene on H/Si(111), a type I-like heterojunction (lowest-energy electron and hole states on Si) is found. For isolated pentacene, the molecular and semiconductor valence band edges are degenerate. For monolayer films, we show how to construct supercell geometries with up to 1192 atoms, which minimize the strain between the inorganic surface and an organic monolayer film. Based on these models, we predict the formation of type II heterojunctions (electron states on Si, hole-like states on the organic species) for both acenes, indicating that charge separation at the interface between the organic and inorganic components is favored. The paper discusses the steps needed to find appropriate low-energy interface geometries for weakly bonded organic molecules and films on inorganic substrates from first principles, a necessary prerequisite for any computational level alignment prediction.

Quantum effects in graphitic materials: Colossal magnetoresistance, Andreev reflections, ferromagnetism, and granular superconductivity

Unlike the more common local conductance spectroscopy, nonlocal conductance can differentiate between nontopological zero-energy modes localized around inhomogeneities, and true Majorana edge modes in the topological phase. In particular, negative nonlocal conductance is dominated by the crossed Andreev reflection. Fundamentally, the effect reflects the system’s topology. In graphene, the Andreev reflection and the inter-band Klein tunneling couple electronlike and hole-like states through the action of either a superconducting pair potential or an electrostatic potential. We are here probing quantum phenomena in modified graphitic samples. Four-point contact transport measurements at cryogenic to room temperatures were conducted using a Quantum Design Physical Property Measurement System. The observed negative nonlocal differential conductance Gdiff probes the Andreev reflection at the walls of the superconducting grains coupled by Josephson effect through the semiconducting matrix. In addition, Gdiff shows the butterfly shape that is characteristic to resistive random-access memory devices. In a magnetic field, the Andreev reflection counters the effect of the otherwise lowered conduction. At low temperatures, the magnetoresistance shows irreversible yet strong giant oscillations that are known to be quantum in nature. In addition, we have found evidence for seemingly granular superconductivity. Thus, graphitic materials show potential for quantum electronics applications, including rectification and topological states.

Phase slips and parity jumps in quantum oscillations of inverted InAs/GaSb quantum wells

We present magnetotransport measurements of a strongly hybridized inverted InAs/GaSb double quantum well. We find that the spin-orbit interaction leads to an appreciable spin-splitting of hole-like states, which form distinct Landau levels in a perpendicular magnetic field. The resulting quantum Hall state is governed by a periodic even and odd total filling arising due to the simultaneous occupation of electron-like and hole-like Landau levels of differing degeneracy. Furthermore, oscillatory charge transfer between all involved subbands leads to discrete phase slips in the usual sequential filling of Landau levels, and coincidentally the phase slips are close to $\pi$. These results shed new insights on the Landau level structure in composite systems and have consequences for interpreting intercepts obtained from index plots, which are routinely employed to investigate the presence of Berry's phase.

Counter-propagating charge transport in the quantum Hall effect regime.

The quantum Hall effect, observed in a two-dimensional (2D) electron gas subjected to a perpendicular magnetic field, imposes a 1D-like chiral, downstream, transport of charge carriers along the sample edges. Although this picture remains valid for electrons and Laughlin's fractional quasiparticles, it no longer holds for quasiparticles in the so-called hole-conjugate states. These states are expected, when disorder and interactions are weak, to harbor upstream charge modes. However, so far, charge currents were observed to flow exclusively downstream in the quantum Hall regime. Studying the canonical spin-polarized and spin-unpolarized v = 2/3 hole-like states in GaAs-AlGaAs heterostructures, we observed a significant upstream charge current at short propagation distances in the spin unpolarized state.

FRW and domain walls in higher spin gravity

We present exact solutions to Vasiliev’s bosonic higher spin gravity equations in four dimensions with positive and negative cosmological constant that admit an interpretation in terms of domain walls, quasi-instantons and Friedman-Robertson-Walker (FRW) backgrounds. Their isometry algebras are infinite dimensional higher-spin extensions of spacetime isometries generated by six Killing vectors. The solutions presented are obtained by using a method of holomorphic factorization in noncommutative twistor space and gauge functions. In interpreting the solutions in terms of Fronsdal-type fields in space-time, a field-dependent higher spin transformation is required, which is implemented at leading order. To this order, the scalar field solves Klein-Gordon equation with conformal mass in (A)dS4. We interpret the FRW solution with de Sitter asymptotics in the context of inflationary cosmology and we expect that the domain wall and FRW solutions are associated with spontaneously broken scaling symmetries in their holographic description. We observe that the factorization method provides a convenient framework for setting up a perturbation theory around the exact solutions, and we propose that the nonlinear completion of particle excitations over FRW and domain wall solutions requires black hole-like states.

Transport signatures of top-gate bound states with strong Rashba–Zeeman effect

We suggest a single-mode spin injection scheme in non-ferromagnetic quantum channels utilizing perpendicular strong Rashba spin-orbit and Zeeman fields. By applying a positive top-gate potential in order to inject electrons from the spin-orbit gap to the low-energy regime, we observe coherent destruction of transport signatures of a hole-like quasi-bound state, an electron-like quasi-bound state, or a hole-like bound state features that are sensitive to the selection of the top-gate length along the transport direction.

Giant Spin-Orbit Splitting in Inverted InAs/GaSb Double Quantum Wells.

Transport measurements in inverted InAs/GaSb quantum wells reveal a giant spin-orbit splitting of the energy bands close to the hybridization gap. The splitting results from the interplay of electron-hole mixing and spin-orbit coupling, and can exceed the hybridization gap. We experimentally investigate the band splitting as a function of top gate voltage for both electronlike and holelike states. Unlike conventional, noninverted two-dimensional electron gases, the Fermi energy in InAs/GaSb can cross a single spin-resolved band, resulting in full spin-orbit polarization. In the fully polarized regime we observe exotic transport phenomena such as quantum Hall plateaus evolving in e^{2}/h steps and a nontrivial Berry phase.

Band-inverted gaps in InAs/GaSb and GaSb/InAs core-shell nanowires.

The [111]-oriented InAs/GaSb and GaSb/InAs core-shell nanowires have been studied by the 8 × 8 Luttinger-Kohn Hamiltonian to search for non-vanishing fundamental gaps between inverted electron and hole bands. We focus on the variations of the band-inverted fundamental gap, the hybridization gap, and the effective gap with the core radius and shell thickness of the nanowires. The evolutions of all the energy gaps with the structural parameters are shown to be dominantly governed by the effect of quantum confinement. With a fixed core radius, a band-inverted fundamental gap exists only at intermediate shell thicknesses. The maximum band-inverted gap found is ~4.4 meV for GaSb/InAs and ~3.5 meV for InAs/GaSb core-shell nanowires, and for the GaSb/InAs core-shell nanowires the gap persists over a wider range of geometrical parameters. The intrinsic reason for these differences between the two types of nanowires is that in the shell the electron-like states of InAs is more delocalized than the hole-like state of GaSb, while in the core the hole-like state of GaSb is more delocalized than the electron-like state of InAs, and both favor a stronger electron-hole hybridization.

Growth and electronic properties of Tb silicide layers on Si(111)

The formation, atomic structure, and electronic properties of Tb silicide layers on the Si(111) surface were studied using scanning tunneling microscopy as well as core-level and angle-resolved photoelectron spectroscopy. For Tb exposures around one monolayer, the formation of a hexagonal TbSi2 monolayer was found, while higher coverages led to the formation of a hexagonal Tb3Si5 multilayer with a 3×3R30° superstructure in the bulk layers. For the monolayer silicide, Si-2p core level spectra show a Fermi level position very close to the conduction band minimum of the silicon substrate, while the Fermi level shifts toward midgap in the multilayer case. The electronic structure of the monolayer is characterized by a Fermi surface consisting of electronlike ellipses around the M¯ points and a holelike state around the Γ¯ point. The effective masses of the band around the M¯ points are strongly anisotropic, with values around 1.45 m0 in the long direction and 0.16 m0 in the short direction of the ellipses. In the case of the multilayer, the ellipses around the M¯ points are less eccentric, and there are indications for Umklapp processes due to the 3×3R30° superstructure in the silicide bulk layers. The overall behavior of Tb is found to be similar to that of other trivalent rare earths on Si(111).

Finger-gate manipulated quantum transport in Dirac materials

We investigate the quantum transport properties of multichannel nanoribbons made of materials described by the Dirac equation, under an in-plane magnetic field. In the low energy regime, positive and negative finger-gate potentials allow the electrons to make intra-subband transitions via hole-like or electron-like quasibound states (QBS), respectively, resulting in dips in the conductance. In the high energy regime, double dip structures in the conductance are found, attributed to spin-flip or spin-nonflip inter-subband transitions through the QBSs. Inverting the finger-gate polarity offers the possibility to manipulate the spin polarized electronic transport to achieve a controlled spin-switch.

Double-finger-gate controlled spin-resolved resonant quantum transport in the presence of a Rashba–Zeeman gap

We investigate double finger gate (DFG) controlled spin-resolved resonant transport properties in an n-type quantum channel with a Rashba–Zeeman (RZ) subband energy gap. By appropriately tuning the DFG in the strong Rashba coupling regime, resonant state structures in conductance can be found that are sensitive to the length of the DFG system. Furthermore, a hole-like bound state feature below the RZ gap and an electron-like quasi-bound state feature at the threshold of the upper spin branch can be found that is insensitive to the length of the DFG system.