Band Mixing Research Articles

Interacting holes in Si and Ge double quantum dots: From a multiband approach to an effective-spin picture

The states of two electrons in tunnel-coupled semiconductor quantum dots can be effectively described in terms of a two-spin Hamiltonian with an isotropic Heisenberg interaction. A similar description needs to be generalized in the case of holes due to their multiband character and spin-orbit coupling, which mixes orbital and spin degrees of freedom and splits $j=3/2$ and $j=1/2$ multiplets. Here we investigate two-hole states in prototypical coupled Si and Ge quantum dots via different theoretical approaches. Multiband $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ and configuration-interaction calculations are combined with entanglement measures in order to thoroughly characterize the two-hole states in terms of band mixing and justify the introduction of an effective spin representation, which we analytically derive a from generalized Hubbard model. We find that, in the weak interdot regime, the ground state and first excited multiplet of the two-hole system display---unlike their electronic counterparts---a high degree of $J$ mixing, even in the limit of purely heavy-hole states. The light-hole component additionally induces $M$ mixing and a weak coupling between spinors characterized by different permutational symmetries.

Magnetism-induced ideal Weyl state in bulk van der Waals crystal MnSb2Te4

We have unveiled a magnetic exchange-induced topological phase transition in a bulk natural van der Waals crystal MnSb2Te4, based on magnetization and magnetotransport measurements and first principles calculations. At the A-type antiferromagnetic ground state, MnSb2Te4 is a topologically trivial insulator with a bandgap of ∼ 42 meV at the Γ point of the Brillouin zone. A small magnetic field less than 1.4 T along the c axis can drive the system into a spin fully polarized state, which hosts only a single pair of Weyl points setting near the Γ point at the Fermi level without other band mixing, supported by both the first principles calculations and the measured anomalous Hall effect. The results would setup an excellent paradigm for the study of interplay between magnetism and nontrivial topology of the electronic band structure.

Colossal switchable photocurrents in topological Janus transition metal dichalcogenides

Nonlinear optical properties, such as bulk photovoltaic effects, possess great potential in energy harvesting, photodetection, rectification, etc. To enable efficient light–current conversion, materials with strong photo-responsivity are highly desirable. In this work, we predict that monolayer Janus transition metal dichalcogenides (JTMDs) in the 1T′ phase possess colossal nonlinear photoconductivity owing to their topological band mixing, strong inversion symmetry breaking, and small electronic bandgap. 1T′ JTMDs have inverted bandgaps on the order of 10 meV and are exceptionally responsive to light in the terahertz (THz) range. By first-principles calculations, we reveal that 1T′ JTMDs possess shift current (SC) conductivity as large as 2300 nm μA V−2, equivalent to a photo-responsivity of 2800 mA/W. The circular current (CC) conductivity of 1T′ JTMDs is as large as ∼104 nm μA V−2. These remarkable photo-responsivities indicate that the 1T′ JTMDs can serve as efficient photodetectors in the THz range. We also find that external stimuli such as the in-plane strain and out-of-plane electric field can induce topological phase transitions in 1T′ JTMDs and that the SC can abruptly flip their directions. The abrupt change of the nonlinear photocurrent can be used to characterize the topological transition and has potential applications in 2D optomechanics and nonlinear optoelectronics.

Band mixing in 96,98Mo isotopes * *Supported by the National Natural Science Foundation of China (11875171, 11675071, 11747318), the U. S. National Science Foundation (OIA-1738287, ACI -1713690), U. S. Department of Energy (DE-SC0005248), the Southeastern Universities Research Association, the China-U. S. Theory Institute for Physics with Exotic Nuclei (CUSTIPEN) (DE-SC0009971), and the LSU–LNNU joint research program (9961)

We use the two lowest weight states to fit E2 strengths connecting the and transitions in Mo. Our results confirm that the and states are maximally mixed, and that the states are weakly mixed in both nuclei. An appropriate Hamiltonian to represent the band mixing is found to be exactly solvable, and its eigenstates can be expressed as the basis vectors in the configuration mixing scheme and interacting boson model. The interacting boson model and coexistence mixing configuration under the solvable methods are suitable models for analyzing the band mixing with high accuracy.

Phononic Thermal Transport in Yttrium Hydrides Allotropes

Room-temperature superconductivity has been attracting increasing attention in recent years. Recent studies have proved the potential of compressed H-rich materials for achieving room-temperature superconductivity. In this paper, we study the phononic thermal transport in the rare earth yttrium hydrides allotropes under 0 GPa, 50 GPa, and 300 GPa by using Boltzmann transport equation. We find that the lattice thermal conductivity of yttrium hydrides increases with the pressure among different allotropes, which is attributed to the increase of bond strength and the decrease of phonon-phonon scattering due to structural compression. Yttrium hydrides structure at high pressure of 300 GPa is the superconducting phase, and has high thermal conductivity around 1360 Wm-1K-1 at room temperature. Comparison of phonon properties with existing high thermal conductivity materials further uncovers the origin for the observed high thermal conductivity. For the zero pressure allotrope, a large number of optical flat bands mix with the low-frequency acoustic phonons, which significantly increases the phonon scattering channel and effectively suppresses the phonon lifetime. As for yttrium hydrides allotropes under 50 GPa and 300 GPa, there are two obvious band gaps in the phonon dispersion relation, and the band gap of the structure at 300 GPa is significantly wider. The occurrence of the band gap effectively inhibits the absorption and emission process of the three-phonon interactions, leading to the decrease of phonon scattering and thus the increase of the phonon lifetime and thermal conductivity at high pressure. Our work reveals the physical mechanism of the thermal transport behaviors in yttrium hydrides structures under different pressures.

Rydberg Series of Dark Excitons in Cu_{2}O.

We demonstrate the Rydberg series of dark excitons, known as paraexcitons, up to the principal quantum number n=6 for the yellow exciton series in Cu_{2}O, using second harmonic generation. Each of these states is optically inactive to all orders, but their observation becomes possible by application of a magnetic field which leads to mixing with the quadrupole-allowed bright excitons, called orthoexcitons, of the same n. The dark parastates are generally located below the bright orthostates, whose energies are increased by the electron-hole exchange interaction, except for n=2, where this order is reversed. This inversion occurs due to band mixing, namely, of the 2S_{y,o} orthoexciton of the yellow series with the 1S_{g,o} orthoexciton of the green exciton series.

Selection rules for the excitation of quantum dots by spatially structured light beams: Application to the reconstruction of higher excited exciton wave functions

Spatially structured light fields applied to semiconductor quantum dots yield fundamentally different absorption spectra than homogeneous beams. In this paper, we theoretically discuss the resulting spectra for different light beams using a cylindrical multipole expansion. For the description of the quantum dots we employ a model based on the effective mass approximation including Coulomb and valence band mixing. The combination of a single spatially structured light beam and state mixing allows all exciton states in the quantum dot to become optically addressable. Furthermore, we demonstrate that the beams can be tailored such that single states are selectively excited, without the need of spectral separation. Using this selectivity, we propose a method to measure the exciton wave function of the quantum dot eigenstate. The measurement goes beyond electron density measurements by revealing the spatial phase information of the exciton wave function. Thereby polarization sensitive measurements are generalized by including the infinitely large spatial degree of freedom.

Multicolor ultralong room-temperature phosphorescence from pure organic emitters by structural isomerism

The current research of ultralong room-temperature phosphorescence (RTP) materials is mainly focused on the lifetimes and efficiencies. The tuning of emission colors is in high demand for various potential applications in optoelectronics, however the corresponding research lags behind. Herein, we report three ultralong RTP molecules based on the carbazole and chromone units through isomerization engineering. By adjusting the linking positions of the carbazole substituents, the ultralong RTP emission can be tuned from blue to red band along with lifetime variation. Detailed experimental and theoretical investigations reveal that the changeable emission color emerges from the mixing of multiple phosphorescence bands and have great relation to the stacking of carbazole and chromone chromophores. These materials were used to construct anti-counterfeiting patterns with diverse afterglow emission colors, presenting a promising application potential in the field of information security.

Elastic and inelastic scattering of neutrinos and weakly interacting massive particles on nuclei

The event rates for WIMP-nucleus and neutrino-nucleus scattering processes, expected to be detected in ton-scale rare-event detectors, are investigated. We focus on nuclear isotopes that correspond to the target nuclei of current and future experiments looking for WIMP- and neutrino-nucleus events. The nuclear structure calculations, performed in the context of the deformed shell model, are based on Hartree-Fock intrinsic states with angular momentum projection and band mixing for both the elastic and the inelastic channels. Our predictions in the high-recoil-energy tail show that detectable distortions of the measured/expected signal may be interpreted through the inclusion of the non-negligible incoherent channels

Influence of Size and Shape Anisotropy on Optical Properties of CdSe Quantum Dots.

We used low-temperature reactions to synthesize different-sized CdSe quantum dots (QDs) capped with fatty-acid and phosphine ligands. From the correlation of high-resolution transmission electron microscopy and X-ray diffraction (XRD) analyses of the synthesized QDs, we observed size-dependent shape anisotropy. In addition, the recorded XRD patterns revealed mixed crystal facets with zinc blende and wurtzite structures in small-sized QDs. Furthermore, from differential absorption (DA) spectra, we extracted the electronic transition energies for different-sized QDs, which were found to be similar to the calculated values of the quantum size levels associated with band mixing of CdSe QDs with a moderate bandgap. We found that the excitonic absorption peaks are increasingly “hidden” with decreasing QD size because of the crystal structure and crystalline quality. The results show good agreement with the obtained diffraction patterns and the estimation errors obtained from the DA spectra.

Hole-phonon interactions in quantum dots: Effects of phonon confinement and encapsulation materials on spin-orbit qubits

Spin-phonon interactions are one of the mechanisms limiting the lifetime of spin qubits made in semiconductor quantum dots. At variance with other mechanisms such as charge noise, phonons are intrinsic to the device and can hardly be mitigated. They set, therefore fundamental limits to the relaxation time of the qubits. Here we introduce a general framework for the calculation of the spin (and charge) transition rates induced by bulk (3D) and strongly confined 1D or 2D phonons. We discuss the particular case of hole spin-orbit qubits described by the 6 bands kp model. We next apply this theory to a hole qubit in a silicon-on-insulator device. We show that spin relaxation in this device is dominated by a band mixing term that couples the holes to transverse acoustic phonons through the valence band deformation potential d, and optimize the bias point and magnetic field orientation to maximize the number of Rabi oscillations Q that can be achieved within on relaxation time T1. Despite the strong spin-orbit coupling in the valence band, the phonon-limited Q can reach a few tens of thousands. We next explore the effects of phonon confinement in 1D and 2D structures, and the impact of the encapsulation materials on the relaxation rates. We show that the spin lifetimes can depend on the structure of the device over micrometer-long length scales and that they improve when the materials around the qubit get harder. Phonon engineering in semiconductor qubits may therefore become relevant once the extrinsic sources of relaxation have been reduced.

Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets.

Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.

Band structure and transport property of graphene/h-BN heterostructure under local potentials

We investigate band structure and transport property of lattice-matched graphene/hexagonal boron nitride (h-BN) heterostructure using the tight-binding approach. It shows that local potentials can significantly modify the band structure and the transport property. A method to individually manipulate the edge states by local potentials is proposed, including shifts and other deformations of edge bands. The two-terminal conductance of each layer is quantized but the interlayer conductance is non-quantized due to band mixing. In addition, we explore the Landau level spectrum in graphene/h-BN nanoribbons under both magnetic field and local potentials. The plateaus-like behavior of the interlayer conductance is observed.

All-Dielectric Meta-Surface for Multispectral Photography by Theta Modulation

The traditional theta modulator encodes input information by superimposing Ronchi sub-gratings, which is extremely easy to cause spatial channel overlap that results in bands mixing. In this case, we present an all-dielectric theta modulation meta-surface with a new encoding method, which separates red, green, blue, and achromatic spatial channels on the focal plane. The meta-surface ensures that the positions of focal points are relatively consistent while focusing energy into the sub-wavelength regions. Our study offers a way to facilitate device miniaturization and system integration, which may have an important application in compact multispectral photography only with one detector.

Electronic and optical properties of highly boron-doped epitaxial Ge/AlAs(001) heterostructures

The impact of elemental boron (B) doping on the structural, optical, and magnetotransport properties of epitaxial Ge/AlAs/GaAs(001) heterostructures, grown by solid-source molecular beam epitaxy, was comprehensively investigated. Cross-sectional transmission electron microscopy analysis revealed atomically abrupt Ge:B/AlAs and AlAs/GaAs heterointerfaces and a lack of observable long-range defect formation or B segregation in the epitaxial Ge:B layer. Spectral broadening observed in the measured temperature-dependent photoluminescence spectra suggested valence band mixing during recombination, implying a splitting of the valence band heavy- and light-hole degeneracy due to residual strain resulting from substitutional B incorporation in the Ge epilayer. Temperature-dependent magnetotransport analysis of the B-doped Ge thin films exhibited the tell-tale signature of antilocalization, indicating observable spin–orbit interaction in the Ge:B system. Moreover, the temperature- and magnetic field-dependent magnetotransport results indicate the presence of single-carrier, p-type conduction in the Ge:B film, further affirming the successful incorporation and activation of B at a high concentration (∼4 × 1019 cm−3) and elimination of parallel conduction via the large-bandgap AlAs buffer. Together, these results provide insights into the effects of heavy doping (via elemental solid-source doping) on Ge-based heterostructures and their feasibility in future electronic and photonic applications.

Topological Electronic States on the Surface of a Strained Gapless Semiconductor

We develop the theory describing the topological electronic states on the surface of a gapless strained semiconductor arisen from the mixing of conduction and valence bands. It follows from the pre ...

Phenomenological Analysis of Characteristics of Rotational Bands in $$^{{158,160}}$$Gd Isotopes

Nonadiabatic effects manifested in the energies of states and the probabilities of electromagnetic transitions in $$^{{158,160}}$$Gd nuclei are studied in the context of a phenomenological model that takes into account Coriolis mixing of low-lying rotational bands. The energies of states of the lowest bands built at the internal $${{2}^{ + }}$$, $${{0}^{ + }}$$, and $${{1}^{ + }}$$ levels and the probabilities of $$E2$$-transitions between them are calculated. The mixing effect noticeably affects the wave functions of the band-heads. The calculated energies and probabilities of $$E2$$-transitions are in general satisfactory agreement with experimental data.

Comparison of first principles and semi-empirical models of the structural and electronic properties of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloys

We present and compare three distinct atomistic models—based on first principles and semi-empirical approaches—of the structural and electronic properties of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloys. Density functional theory calculations incorporating Heyd–Scuseria–Ernzerhof (HSE), local density approximation (LDA) and modified Becke–Johnson (mBJ) exchange-correlation functionals are used to perform structural relaxation and electronic structure calculations for a series of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloy supercells. Based on HSE calculations, a semi-empirical valence force field (VFF) potential and $$sp^{3}s^{*}$$ tight-binding (TB) Hamiltonian are parametrised. Comparing the HSE, LDA+mBJ and VFF+TB models, and using the HSE results as a benchmark, we demonstrate that: (1) LDA+mBJ calculations provide an accurate first principles description of the electronic structure at reduced computational cost, (2) the VFF potential is sufficiently accurate to circumvent the requirement to perform first principles structural relaxation, and (3) VFF+TB calculations provide a good quantitative description of the alloy electronic structure in the vicinity of the band edges. Our results also emphasise the importance of Sn-induced band mixing in determining the nature of the conduction band structure of $$\hbox {Ge}_{1-x}\hbox {Sn}_{x}$$ alloys. The theoretical models and benchmark calculations we present inform and enable predictive, computationally efficient and scalable atomistic calculations for disordered alloys and nanostructures. This provides a suitable platform to underpin further theoretical investigations of the properties of this emerging semiconductor alloy.

Transport characteristics of multi-terminal pristine and defective phosphorene systems

Atomic vacancies and nanopores act as local scattering centers and modify the transport properties of charge carriers in phosphorene nanoribbons (PNRs). We investigate the influence of such atomic defects on the electronic transport of multi-terminal PNR. We use the non-equilibrium Green’s function approach within the tight-binding framework to calculate the transmission coefficient and the conductance. Terminals induce band mixing resulting in oscillations in the conductance. In the presence of atomic vacancies and nanopores the conductance between non-axial terminals exhibit constructive scattering, which is in contrast to mono-axial two-terminal systems where the conductance exhibits destructive scattering. This can be understood from the spatial local density of states of the transport modes in the system. Our results provide fundamental insights into the electronic transport in PNR-based multi-terminal systems and into the ability of atomic defects and nanopores through tuning the transport properties.

Hyperfine interaction for holes in quantum dots: k·p model

We formulate the multi-band kp theory of hyperfine interactions for semiconductor nanostructures in the envelope function approximation. We apply this theoretical description to the fluctuations of the longitudinal and transverse Overhauser field experienced by a hole for a range of InGaAs quantum dots of various compositions and geometries. We find that for a wide range of values of $d$-shell admixture to atomic states forming the top of the valence band, the transverse Overhauser field caused by this admixture is of the same order of magnitude as the longitudinal one, and band mixing adds only a minor correction to this result. In consequence, the kp results are well reproduced by a simple box model with the effective number of ions determined by the wave function participation number, as long as the hole is confined in the compositionally uniform volume of the dot, which holds in a wide range of parameters, excluding very flat dots.