The spin filtering effect in the bilayer nanowire with quantum point contact is investigated theoretically. We demonstrate the new mechanism of the spin filtering based on the lateral inter-subband spin-orbit coupling, which for the bilayer nanowires has been reported to be strong. The proposed spin filtering effect is explained as the joint effect of the Landau-Zener inter-subband transitions caused by the hybridization of states with opposite spin (due to the lateral Rashba SO interaction) and the confinement of carriers in the quantum point contact region.
A longstanding goal of spintronics is to inject, coherently transport, and detect spins in a semiconductor nanowire where a SINGLE quantized subband is occupied at room temperature. Here, we report achieving this goal in 50-nm diameter InSb nanowires by demonstrating both the spin-valve and the Hanle effect. The spin relaxation time in the nanowires was found to have increased by an order of magnitude over what has been reported in bulk and quantum wells due to the suppression of D'yakonov-Perel' spin relaxation as a result of single subband occupancy. These experiments raise hopes for the rea…
Subband occupancy (i.e. the number of occupied subbands or energy levels in the semiconductor) is a key physical parameter characterizing the topological properties of superconductor-semiconductor hybrid systems in the context of the search for non-Abelian Majorana zero modes. We theoretically study the subband occupation of semiconductor nanowire devices as function of the applied gate potential, the semiconductor-superconductor (SM-SC) work function difference, and the surface charge density by solving self-consistently the Schrödinger-Poisson equations for the conduction electrons of the se…
By means of a multiband effective mass Hamiltonian, a theoretical characterization of the effect of the geometrical features of the confinement profile --in particular, a longitudinal groove-- on the subbands dispersion and spin-related properties of a rectangular HgTe/CdTe nanowire is presented. Through an external electric field applied perpendicular to the wire, the interplay of the induced Rashba spin splitting and these geometrical features is investigated. It is found that by exploiting this interplay a rich complexity of the subband structure arises, permitting the generation and modula…
We report on magneto-transport measurements in InAs nanowires under large magnetic field (up to 55T), providing a direct spectroscopy of the 1D electronic band structure. Large modulations of the magneto-conductance mediated by an accurate control of the Fermi energy reveal the Landau fragmentation, carrying the fingerprints of the confined InAs material. Our numerical simulations of the magnetic band structure consistently support the experimental results and reveal key parameters of the electronic confinement.
We report on possibility of charge current generation in nanowire made of two tunnel coupled one-dimensional electron waveguides by means of single magnetic pulse lasting up to 20 ps. Existence of interlayer tunnel coupling plays a crucial role in the effect described here as it allows for hybridization of the wave functions localized in different layers which can be dynamically modified by applying a time changeable in-plane magnetic field. Results of time-dependent DFT calculations performed for a bilayer nanowire confining many electrons show that the effect of such magnetic hybridization r…
We report a study of one-dimensional subband splitting in a bilayer graphene quantum point contact in which quantized conductance in steps of $4\,e^2/h$ is clearly defined down to the lowest subband. While our source-drain bias spectroscopy measurements reveal an unconventional confinement, we observe a full lifting of the valley degeneracy at high magnetic fields perpendicular to the bilayer graphene plane for the first two lowest subbands where confinement and Coulomb interactions are the strongest and a peculiar merging/mixing of $K$ and $K'$ valleys from two non-adjacent subbands with indi…
Strong `spin'-orbit coupled one-dimensional hole gas is achievable in a Ge nanowire in the presence of a strong magnetic field. The strong magnetic field lifts the two-fold degeneracy in the hole subband dispersions, so that the effective low-energy subband dispersion exhibits strong `spin'-orbit coupling. Here, we study the electrical `spin' manipulation in a Ge nanowire quantum dot for both the lowest and second lowest hole subband dispersions. Using a finite square well to model the quantum dot confining potential, we calculate exactly the level splitting of the `spin'-orbit qubit and the R…
Full-shell nanowires (semiconducting nanowires fully coated with a superconducting shell) have been recently presented as a novel means to create Majorana zero modes. In contrast to partially coated nanowires, it has been argued that full-shell nanowires do not require high magnetic fields and low densities to reach a putative topological regime. Here we present a theoretical study of these devices taking into account all the basic ingredients, including a charge distribution spread across the section of the nanowire, required to qualitatively explain the first experimental results (Vaitiekena…
Spin-3/2 physics of semiconductor hole nanowires: Valence-band mixing and tunable interplay between bulk-material and orbital bound-state spin splittings
We demonstrate that, at the onset of conduction, an electrostatically defined quantum wire in bilayer graphene (BLG) with an interlayer asymmetry gap may act as a 1D semimetal, due to the multiple minivalley dispersion of its lowest subband. Formation of a non-monotonic subband coincides with a near-degeneracy between the bottom edges of the lowest two subbands in the wire spectrum, suggesting an $8e^2/h$ step at the conduction threshold, and the semimetallic behaviour of the lowest subband in the wire would be manifest as resonance transmission peaks on an $8e^2/h$ conductance plateau.
The radial confining potential in a semiconductor nanowire plays a key role in determining its quantum transport properties. Previous reports have shown that an axial magnetic field induces flux-periodic conductance oscillations when the electronic states are confined to a shell. This effect is due to the coupling of orbital angular momentum to the magnetic flux. Here, we perform calculations of the energy level structure, and consequently the conductance, for more general cases ranging from a flat potential to strong surface band bending. The transverse states are not confined to a shell, but…
We report an experimental study of one-dimensional (1D) electronic transport in an InSb semiconducting nanowire. Three bottom gates are used to locally deplete the nanowire creating a ballistic quantum point contact with only a few conducting channels. In a magnetic field, the Zeeman splitting of the corresponding 1D subbands is revealed by the emergence of conductance plateaus at multiples of $e^2$/h, yet we find a quantized conductance pattern largely dependent on the configuration of voltages applied to the bottom gates. In particular, we can make the first plateau disappear leaving a first…
We performed measurements at helium temperatures of the electronic transport in an InAs quantum wire ($R_{wire} \sim 30$\,k$Ω$) in the presence of a charged tip of an atomic force microscope serving as a mobile gate. The period and the amplitude of the observed quasiperiodic oscillations are investigated in detail as a function of electron concentration in the linear and non-linear regime. We demonstrate the influence of the tip-to-sample distance on the ability to locally affect the top subband electrons as well as the electrons in the disordered sea. Furthermore, we introduce a new method of…
Spintronic devices usually rely on long spin relaxation times and/or lengths for optimum performance. Therefore, the ability to modulate these quantities with an external agent offers unique possibilities. The dominant spin relaxation mechanism in most technologically important semiconductors is the D'yakonov-Perel' (DP) mechanism which vanishes if the spin carriers (electrons) are confined to a single conduction subband in a quantum wire grown in certain crystallographic directions, or polycrystalline quantum wires. Here, we report modulating the DP spin relaxation rate (and hence the spin re…
A heterostructure consisting of a topological insulator (TI) nanowire brought into proximity with a superconducting layer provides a promising route to achieve topological superconductivity and associated Majorana bound states (MBSs). Here, we study effects caused by such a coupling between a thin layer of an $s$-wave superconductor and a TI nanowire. We show that there is a distinct phenomenology arising from the metallization of states in the TI nanowire by the superconductor. In the strong coupling limit, required to induce a large superconducting pairing potential, we find that metallizati…
Quantized Conductance through Surface States in High Quality Three-Dimensional Dirac Semimetal Cd$_3$As$_2$ Nanowire/Nanoribbon p-n Junctions
DGIST Research Institute, DGIST（大邱科学技术院研究学院）；Electrical and Computer Engineering, National University of Singapore（新加坡国立大学电气与计算机工程系）；Department of Physics, KTH Royal Institute of Technology（皇家理工学院物理系）；Industrial AX Innovation Research Institute and Sensor Technology Support Center, DGIST（大邱科学技术院工业AX创新研究所及传感器技术支持中心）；Department of Advanced Materials Chemistry, Korea University（韩国大学先进材料化学系）；Department of Physics and Chemistry, DGIST（大邱科学技术院物理化学系）；Department of Interdisciplinary Engineering, DGIST（大邱科学技术院交叉工程学系）；Institute of next generation semiconductor technology (INST), DGIST（大邱科学技术院下一代半导体技术研究所）
We study longitudinal magnetotransport in disorder-free cylindrical Weyl semimetal nanowires. Our theory includes a magnetic flux $Φ$ piercing the nanowire and captures the finite curvature of the Fermi arc in the surface Brillouin zone through a boundary angle $α$. Electron backscattering by acoustic phonons via the deformation potential causes a finite resistivity which we evaluate by means of the semiclassical Boltzmann approach. We find that low-energy transport is dominated by surface states, where transport observables are highly sensitive to the angle $α$ and to Aharonov-Bohm phases due…
We simulate the electron transport in vertical bi-layer nanowire which can be fabricated in molecular beam epitaxy process with lateral confinement potential formed by means of cleaved overgrowth or surface oxidization methods giving rectangular and smooth side confinement, respectively. In calculations we take into account interaction between charge carriers using DFT within local spin density approximation. If magnetic field is perpendicular to the wire axis, pseudogaps are opened in energy dispersion relation E(k) what in conjunction with spin Zeeman shift of spin-up and spin-down subbands …
We study semiconductor nanowires coupled to a bilayer of a disordered superconductor and a magnetic insulator, motivated by recent experiments reporting possible Majorana-zero-mode signatures in related architectures. Specifically, we pursue a quasiclassical Usadel equation approach that treats superconductivity in the bilayer self-consistently in the presence of spin-orbit scattering, magnetic-impurity scattering, and Zeeman splitting induced by both the magnetic insulator and a supplemental applied field. Within this framework we explore prospects for engineering topological superconductivit…
We report on measurements of quantized conductance in gate-defined quantum point contacts in bilayer graphene that allow the observation of subband splittings due to spin-orbit coupling. The size of this splitting can be tuned from 40 to 80 $μ$eV by the displacement field. We assign this gate-tunable subband-splitting to a gap induced by spin-orbit coupling of Kane-Mele type, enhanced by proximity effects due to the substrate. We show that this spin-orbit coupling gives rise to a complex pattern in low perpendicular magnetic fields, increasing the Zeeman splitting in one valley and suppressing…
We present a simple model to describe the lowest-subbands surface scattering in locally oxidized silicon nanowires grown in the [110] direction. To this end, we employ an atomistically scaled effective mass model projected from a three-dimensional effective mass equation and apply a quantum transport formalism to calculate the conductance for typical potential profiles. Comparison of our results with hole-transport calculations using atomistic models in conjunction with density functional theory (DFT) points to an intra-subband scattering mechanism from a potential well.
We constructed the Hamiltonian of spin-orbit splitting for carriers of a tubular electron gas in InAs nanowires. The spectral problem is solved using an exact numerical diagonalization. It is shown that the contribution of k-linear Dresselhaus-like spin-orbit (SO) coupling leads to renormalization of so-called SO-gaps and appearance of anticrossings in subband spectrum. These features can be detected in ballistic transport.
The magnetoelectronic properties of quasi-one-dimensional zigzag graphene nanoribbons are investigated by using the Peierls tight-binding model. Quasi-Landau levels (QLLs), dispersionless Landau subbands within a certain region of k-space, are resulted from the competition between magnetic and quantum confinement effects. In bilayer system, the interlayer interactions lead to two groups of QLLs, one occurring at the Fermi level and the other one occurring at higher energies. Transverse electric fields are able to distort energy spectrum, tilt two groups of QLLs and cause semiconductor-metal tr…
We demonstrate that the Fulde-Ferrell (FF) phase can be induced uniquely by the orbital effect in a cylindrical metallic nanowire. In the external magnetic field the two-fold degeneracy with respect to the orbital quantum number $m$ is lifted what leads to a Fermi wave vector mismatch between the subbands with opposite orbital momenta in the paired state. This mismatch can be compensated by the nonzero total momentum of the Cooper pairs created by electrons from split subbands what results in the formation of the FF phase. With increasing magnetic field a series of FF stability regions appear …
We study ballistic hole transport through Ge/Si core/shell nanowires at low temperatures. We observe Fabry-P$\acute{e}$rot interference patterns as well as conductance plateaus at integer multiples of 2e$^2$/h at zero magnetic field. Magnetic field evolution of these plateaus reveals large effective Land$\acute{e}$ g-factors. Ballistic effects are observed in nanowires with silicon shell thicknesses of 1 - 3 nm, but not in bare germanium wires. These findings inform the future development of spin and topological quantum devices which rely on ballistic subband-resolved transport.
Three-dimensional topological insulator (TI) nanowires with quantized surface subband spectra are studied as a main component of Majorana bound states (MBS) devices. However, such wires are known to have large concentration $N \sim 10^{19}$ cm$^{-3}$ of Coulomb impurities. It is believed that a MBS device can function only if the amplitude of long-range fluctuations of the random Coulomb potential $Γ$ is smaller than the subband gap $Δ$. Here we calculate $Γ$ for recently experimentally studied large-dielectric-constant (Bi$_{1-x}$Sb$_x$)$_2$Te$_{3}$ wires in a small-dielectric-constant enviro…
We present a study on the lifting of degeneracy of the size-quantized energy levels in an electrostatically defined quantum point contact in bilayer graphene by the application of in-plane magnetic fields. We observe a Zeeman spin splitting of the first three subbands, characterized by effective Landé $g$-factors that are enhanced by confinement and interactions. In the gate-voltage dependence of the conductance, a shoulder-like feature below the lowest subband appears, which we identify as a $0.7$ anomaly stemming from the interaction-induced lifting of the band degeneracy. We employ a phenom…
In bilayer graphene rotationally faulted to theta=1.1 degrees, interlayer tunneling and rotational misalignment conspire to create a pair of low energy flat band that have been found to host various correlated phenomena at partial filling. Most work to date has focused on the zero magnetic field phase diagram, with magnetic field (B) used as a probe of the B=0 band structure. Here, we show that twisted bilayer graphene (tBLG) in a B as low as 2T hosts a cascade of ferromagnetic Chern insulators with Chern number |C|=1,2 and 3. We argue that the emergence of the Chern insulators is driven by th…
The theoretical description has been proposed for the operation of the spin transistor in the gate-controlled InAs nanowire. The calculated current-voltage characteristics show that the current flowing from the source (spin injector) to the drain (spin detector) oscillates as a function of the gate voltage, which results from the precession of the electron spin caused by the Rashba spin-orbit interaction in the vicinity of the gate. We have studied two operation modes of the spin transistor: (A) the ideal operation mode with the full spin polarization of electrons in the contacts, the zero tem…
In this paper, we focus on the performance of a nanowire Field Effect Transistor (FET) in the Ultimate Quantum Capacitance Limit (UQCL) (where only one subband is occupied) in the presence of interface traps ($D_{it}$), parasitic capacitance ($C_L$) and source/drain series resistance ($R_{s,d}$) using a ballistic transport model and compare the performance with its Classical Capacitance Limit (CCL) counterpart. We discuss four different aspects relevant to the present scenario, namely, (i) gate voltage dependent capacitance, (ii) saturation of the drain current, (iii) the subthreshold slope an…
We observed broken-symmetry quantum Hall effects and level crossings between spin- and valley- resolved Landau levels (LLs) in Bernal stacked trilayer graphene. When the magnetic field was tilted with respect to sample normal from $0^{\circ}$ to $66^\circ$, the LL crossings formed at intersections of zeroth and second LLs from monolayer-graphene-like and bilayer-graphene-like subbands, respectively, exhibited a sequence of transitions. The results indicate the LLs from different subbands are coupled by in-plane magnetic fields ($B_{\parallel}$), which was explained by developing the tight-bind…
We study the nuclear spin relaxation in a ballistic nanowire with hyperfine and Rashba spin-orbit interactions (SOI) and in the presence of magnetic field and electron interactions. The relaxation rate shows pronounced peaks as function of magnetic field and chemical potential due to van Hove singularities in the Rashba bands. As a result, the regimes of weak and strong SOIs can be distinguished by the number of peaks in the rate. The relaxation rate increases with increasing magnetic field if both Rashba subbands are occupied, whereas it decreases if only the lowest one is occupied.
A key concept in the field of semiconductor spintronics is an electric field control of spins via the spin-orbit coupling (SOC) and the SOC strength governs efficiency of this control. We propose a new approach that allows the experimental determination of the Rashba SOC strength for ballistic InAs nanowires. The energy spectrum and ballistic transport of carriers through the nanowire with surface two-dimensional electron gas (2DEG) in a homogeneous magnetic field are studied. A general formula for the linear-response one-dimensional ballistic thermopower is derived in the case of complex subb…
For superconducting nanowire with the pairing of extended s-type symmetry, Rashba spin-orbit interaction in a magnetic field, the influence of strong intersite charge correlations on single-particle Majorana excitations is analyzed. This problem is investigated on the basis of the density matrix renormalization group numerical method. It is shown that with an increase in the repulsion intensity of electrons located at the neighboring sites, two subbands emerge in the lower Hubbard band of the open system. Based on calculations of the Majorana polarization and degeneracy of the entanglement spe…
Mechanical properties, atomic and energy band structures of bare and hydrogen passivated Si$_{n}$Ge$_{n}$ nanowire superlattices have been investigated by using first-principles pseudopotential plane wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. We found that Si$_{n}$ nanowires are stiffer than Ge$_{n}$ nanowires. Hydrogen passivation makes these nanowires and Si$_{n}$Ge$_{n}$ nanowire superlattice even more stiff. Upon heterostructure formation, superlattice electronic states form subbands in mom…
Giant and robust Josephson diode effect in multiband topological nanowires
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料国际中心）；Hefei National Laboratory（合肥国家实验室）；International Quantum Academy, Shenzhen（深圳国际量子研究院）
By employing a Boltzmann transport equation and using an energy and size dependent relaxation time ($τ$) approximation (RTA), we evaluate self-consistently the thermoelectric figure-of-merit $ZT$ of a quantum wire with rectangular cross-section. The inferred $ZT$ shows abrupt enhancement in comparison to its counterparts in bulk systems. Still, the estimated $ZT$ for the representative Bi$_2$Te$_3$ nanowires and its dependence on wire parameters deviate considerably from those predicted by the existing RTA models with a constant $τ$. In addition, we address contribution of the higher energy su…
We have studied the edge spin accumulation in a high mobility two-dimensional electron gas formed in a symmetric well with two subbands. This study is strongly motivated by the recent experiment of Hernandez et al. [Phys. Rev. B {\bf 88}, 161305(R) (2013)] who demonstrated the spin accumulation near the edges of a bilayer symmetric GaAs structure in contrast to no effect in a single-layer configuration. The intrinsic mechanism of the spin-orbit interaction we consider arises from the coupling between two subband states of opposite parities. We obtain a parametrically large magnitude of the edg…
A semiconductor nanowire based superconductor-normal-superconductor (SNS) junction is modeled theoretically. A magnetic field is applied along the nanowire axis, parallel to the current. The Bogoliubov-de Gennes equations for Andreev bound states are solved while considering the electronic subbands due to radial confinement in the N-section. The energy-versus-phase curves of the Andreev bound states shift in phase as the N-section quasiparticles with orbital angular momentum couple to the axial field. A similar phase shift is observed in the continuum current of the junction. The quantum mecha…
Spin-momentum locking in a semiconductor device with strong spin-orbit coupling (SOC) is a fundamental goal of nanoscale spintronics and an important prerequisite for the formation of Majorana bound states. Such a helical state is predicted in one-dimensional (1D) nanowires subject to strong Rashba SOC and spin-mixing, its hallmark being a characteristic reentrant behaviour in the conductance. Here, we report the first direct experimental observations of the reentrant conductance feature, which reveals the formation of a helical liquid, in the lowest 1D subband of an InAs nanowire. Surprisingl…
