Topological Nanowires Research Articles

Dual Negative Differential of Heat Generation in a Strongly Correlated Quantum Dot Side-Coupled to Majorana Bound States

We study theoretically the properties of local heat originated from energy exchange between electrons passing through a quantum dot (QD) coupled to a phonon bath. The dot is sandwiched between two normal metal leads and also side-coupled to Majorana bound states (MBSs) formed at opposite ends of a topological superconductor nanowire. We find that in addition to the negative differential of heat generation (NDHG) in the Coulomb blockade regime, another NDHG emerges near the leads’ Fermi level due to the dot-MBS coupling. This dual NDHG effect is robust against the variation of intradot Coulomb interaction strength, and disappears if the QD is coupled to regular Fermions. Direct hybridization between the MBSs reduces their impacts on the electronic transport processes, and eliminates the dual NDHG effect. Our results show that the dual NDHG effect is quite efficient for inferring the existence of MBSs, and may remedy some limitations of the detection schemes relying on tunneling spectroscopy technique.

1D topological systems for next-generation electronics

1D topological systems for next-generation electronics

Photon-Assisted Average Current Through a Quantum Dot Coupled to Majorana Bound States

We investigate the properties of photon-assisted current through a quantum dot (QD) coupled to two leads and Majorana bound states (MBSs) formed at two ends of a topological superconductor nanowire. Our numerical results show that the peak’s height of the average current is reduced to half by the half fermionic character of MBSs. When a microwave (MW) field is applied on one of the leads, the negative current is converted into positive by the MBSs. This abnormal phenomenon may serve as a detection method for MBSs. Moreover, the photon-assisted tunneling sideband peak appears while a MW field is applied on the QD. Turning on the coupling between the QD and MBSs, the magnitude of the average current decreases monotonously because that the MBSs now is changed to the non-zero energy mode by emitting (absorbing) photons from the MW field. Correspondingly, the energy level in the QD is split into two by the non-zero energy mode MBSs, and then the photon-assisted tunneling occurs with the help of the MBSs.

Majorana Bound States Hallmarks in a Quantum Topological Interferometer Ring

AbstractIn this work, the conductance and current correlations properties of a quantum topological interferometer consisting of a quantum dot coupled to two Majorana bound states (MBSs) confined at both ends of a 1D topological superconductor ring nanowire is investigated. The ring in its topological nontrivial and trivial phases is analyzed to show that the tunneling conductance, shot noise, and Fano factor present unique characteristics to distinguish the hallmark of MBSs. The findings are reinforced by taking advantage of the full correspondence between the quantum topological interferometer and a quantum‐dot effectively coupled to a single Majorana state in a straight topological superconductor wire configuration. It is shown that besides the characteristic zero‐bias conductance and the already known shot noise features, the Fano factor provides significant information to distinguish the MBSs presence.

Photon-Assisted Seebeck Effect in a Quantum Dot Coupled to Majorana Zero Modes

The Seebeck effect is investigated within the framework of a non-equilibrium Green's function technique in a quantum dot (QD) sandwiched between the left and right leads held at different temperatures. We consider that the QD is shelled by a photon field and is side-coupled to a topological superconductor nanowire hosting Majorana zero modes (MZMs). It is found that the thermopower (Seebeck coefficient) can be obviously enhanced by weak QD-MZMs coupling at low temperatures, in addition to its sign reversion that may be used for detecting the existence of MZMs. In the presence of a photon field, the thermopower can be further enhanced due to decreased electrical conductance when electrons' transport probability through each channel is reduced by photon-assisted tunneling (PAT). The hybridization between the MZMs will also induce sign reversion of the thermopower in the absence of a photon field, whereas it has less impact on the thermopower when the QD is shelled by the photon field.

Thermoelectric Transport in a Double-Quantum-Dot Coupled to Majorana Zero Modes

Thermoelectric transport through a double-quantum-dot (DQD) connected to the left and right leads is theoretically investigated in the framework of non-equilibrium Green’s function technique. We consider that the dots are also coupled to Majorana zero modes (MZMs) prepared at the two ends of a topological superconductor nanowire. It is found that the sign change of thermopower, which is promising in the detection of MZMs, can be realized by tuning several system’s parameters related to the MZMs, such as the coupling strength between the dots and the MZMs, the direct coupling between the MZMs, or even the magnetic flux penetrating through the structure. The above parameters also lead to significant enhancement of the thermopower and thermoelectric figure of merit (FOM), which indicates the conversion efficiency between heat and electrical energies. We also find that in this DQD system, both the thermopower and FOM are simultaneously enhanced by the MZMs around the electron-hole symmetric point, an ideal phenomenon in applications of thermoelectric effect. In addition, the thermoelectric effect is remarkably enhanced by the direct hybridization between the MZMs, which is very different from the case in single-dot structure.

Quantum Interference Effects in Quantum Dot Molecular With Majorana Bound States

Non-invasive detection of the Majorana bound state (MBSs), a kind of quasiparticle without charge and mass, is one of the core issues in current condensed matter physics. Here we study in theory the quantum interference effect in parallel-coupled double quantum dots which are connected either by Majorana bound states (MBSs) or regular fermions. We find that the zero-energy conductance develops a sharp peak when the dots are connected by the MBSs, whereas that in the case of the dots are coupled via regular fermions shows a valley. By varying the coupling strength between the dots and the electron reservoirs, the conductance in the two structures changes in different ways. By comparing the properties of the linear conductance in the two systems, the information of the MBSs formed at the two ends of a topological superconductor nanowire then can be inferred. We also find that the MBSs in the present structure also induces the Fano effect, and is favorable in quantum information processing.

Electronic Transport Through Double Quantum Dot Coupled to Majorana Bound States and Ferromagnetic Leads

We have studied theoretically the properties of electrical current and tunnel magnetoresistance (TMR) through a serially connected double quantum dot (DQD) sandwiched between two ferromagnetic leads by using the nonequilibrium Green’s function technique. We consider that each of the DQD couples to one mode of the Majorana bound states (MBSs) formed at the ends of a topological superconductor nanowire with spin-dependent coupling strength. By adjusting the sign of the spin polarization of dot–MBS coupling strength and the arrangement of magnetic moments of the two leads, the currents’ magnitude can be effectively enhanced or suppressed. Under some conditions, a negative TMR emerges which is useful in detection of the MBSs, a research subject currently under extensive investigations. Moreover, the amplitude of the TMR can be adjusted in a large regime by variation of several system parameters, such as direct hybridization strength between the MBSs or the dots and the positions of the dots’ energy levels. Such tunable currents and TMR may also find use in high-efficiency spintronic devices or information processes.

The impact of metallic contacts on spin-polarized photocurrents in topological insulator Bi2Se3 nanowires

Recently, a quantum phase, the topological insulator, has been vividly investigated in a variety of materials. Its unique band structure allows for optical generation and control of spin-polarized currents based on the circular photogalvanic effect. In this paper, we generate and distinguish the different photocurrent contributions via the polarization of the driving light wave. We discuss the helicity-dependent spin-polarized current and the polarization-independent thermoelectric current as spatially resolved maps, focusing on the influence of the topological insulator/metallic contact interface. We observe for both current contributions a significant enhancement of the current values at the topological insulator/metallic contact interface. In the case of the thermoelectric current, the enhancement is localized at the center of the interface. The spin-polarized current reaches two extrema per contact, which differ by their sign and are localized nearby the contact edges. We discuss the general behavior of the thermovoltage as a three-material Seebeck effect and explain the enhanced values by the acceleration of the photoelectrons generated in the space charge region of the topological insulator/metallic contact interface. Furthermore, we interpret the temperature gradient together with the spin Nernst effect as a possible origin for the enhancement and spatial distribution of the spin-polarized current.

A topological Josephson junction platform for creating, manipulating, and braiding Majorana bound states

As part of the intense effort toward identifying platforms in which Majorana bound states can be realized and manipulated to perform qubit operations, we propose a topological Josephson junction architecture that achieves these capabilities and which can be experimentally implemented. The platform uses conventional superconducting electrodes deposited on a topological insulator film to form networks of proximity-coupled lateral Josephson junctions. Magnetic fields threading the network of junction barriers create Josephson vortices that host Majorana bound states localized in the junction where the local phase difference is an odd multiple of π, i.e. attached to the cores of the Josephson vortices. This enables us to manipulate the Majorana states by moving the Josephson vortices, achieving functionality exclusive to these systems in contrast to others, such as those composed of topological superconductor nanowires. We describe protocols for: (1) braiding localized Majorana states by exchange, (2) controlling the separation and hence the coupling of adjacent localized Majorana states to effect non-Abelian rotations via hybridization of the Majorana modes, and (3) reading out changes in the non-local parity correlations induced by such operations. These schemes utilize current pulses and local magnetic field pulses to control the location of vortices, and measurements of the Josephson current–phase relation to reveal the presence of the Majorana bound states. Finally, we present brief discussions of readout schemes and viable experimental settings for realizing the platform.

Critical current for an insulating regime of an underdamped current-biased topological Josephson junction

We study analytically an underdamped current-biased topological Josephson junction. First, we consider a simplified model at zero temperature, where the parity of the non-local fermionic state formed by Majorana bound states (MBSs) localized on the junction is fixed, and show that a transition from insulating to conducting state in this case is governed by single-quasiparticle tunneling rather than by Cooper pair tunneling in contrast to a non-topological Josephson junction. This results in a significantly lower critical current for the transition from insulating to conducting state. We propose that, if the length of the system is finite, the transition from insulating to conducting state occurs at exponentially higher bias current due to hybridization of the states with different parities as a result of the overlap of MBSs localized on the junction and at the edges of the topological nanowire forming the junction. Finally, we discuss how the appearance of MBSs can be established experimentally by measuring the critical current for an insulating regime at different values of the applied magnetic field.

Leakage of Majorana bound states in an inhomogeneous topological nanowire

We present an exact solution of the continuum Bogolyubov-de-Gennes Hamiltonian for Majorana bound states (MBSs) generated in a superconductor–semiconductor hybrid topological nanowire. The full energy spectra that include the band states and in-gap states could be obtained. We show that for relatively short wire length, the zero energy mode could be induced even in the topological trivial regime, which also indicates oscillatory dependence on the chemical potential. With the increase of the Zeeman field, the MBSs are almost fully spin-polarized and do not localize at the wire ends gradually. We also extend our discussion to the property of Majorana modes in an inhomogeneous nanowire, in which a local gate voltage is applied to one end of the nanowire. It is found that the local potential barrier or well could modulate the Majorana energy splitting periodically. The leakage of MBSs to the potential region is exponentially suppressed for the barrier case. A potential well could induce near-zero-energy bound states and these states merge with MBSs, leading to the delocalization of MBSs. In the potential well region, both the spin-up and spin-down components of the trivial states account for a significant proportion, which can be detected experimentally.

Photon-Assisted Transport Through a Quantum Dot Side-Coupled to Majorana Bound States

Transmission function of a system composing of a quantum dot (QD) subjected to a photon field and side-coupled to a topological superconductor nanowire hosting a pair of Majorana bound states (MBSs) is calculated by using the nonequilibrium Green's function technique. We find that a series of photon-induced peaks emerge and are split by the coupling between the QD and the MBSs. Moreover, the peaks' height are suppressed to zero because the MBSs absorb (emit) the photon energy. Under this condition, the MBSs may be shifted to the non-zero energy mode, and thus provide another detection scheme for its existence which is quite different from the currently adopted ones depending on the zero-energy mode of the MBSs. In the presence of MBS-MBS overlaping, the central photon-assisted peaks in the transmission function reappear due to the fact that the photon absorbed (emitted) by one mode of the MBSs are subsequently emitted (absorbed) by another MBSs' mode. We also find that the positions of the additional peaks induced by the MBS-MBS overlaping in the presence of the photon field are quite different from the case of zero photon field.

Signatures of Majorana bound state in an Aharonov-Bohm interferometer embedded with quantum dot

Abstract We consider a ring geometry with a quantum dot (QD) embedded in one of its arms, which is coupled symmetrically to two normal leads and threaded by Aharonov-Bohm (AB) flux φ. The QD is coupled to a topological nanowire. Using scattering matrix method, the differential conductance and current cross correlation are investigated. By varying the external magnetic field strength and the coupling strength between the QD and Majorana bound state (MBS), the zero-bias conductance peak (ZBCP) will split and oscillate due to the overlapping of the two MBSs. Due to the sum rule in a pair of well-separated MBSs, the total differential conductance at zero bias voltage is quantized to 2e2/h. The total differential conductance and the current cross correlation show the periodicity of π with zero QD energy level and 2π with nonzero QD energy level.

Distinguishing between topological and quasi Majorana zero modes with a dissipative resonant level

Distinguishing between topological Majorana zero modes and quasi Majorana modes -- trivial low energy Andreev bound states -- is an important step towards realizing topological hybrid nanowire devices. We propose that this distinction can be made by connecting a hybrid nanowire to a dissipative resonant level realized by a quantum dot weakly coupled to high resistance leads. We show that the qualitative temperature scaling of the width of the resonant level zero bias conductance peak provides a signature unique to true Majorana zero modes. The degeneracy induced by a true Majorana zero mode is further shown to stabilize this conductance peak against particle-hole asymmetry.

The modulation of Majorana bound states comb through quantum dots

Performing the scattering matrix approach, we calculate the current noise cross correlation and the differential conductance in the Majorana bound states (MBSs) comb. The teeth on the comb consist of topological super-conducting nanowire sandwiched between two quantum dots (QDs). We find that the nonlocal property of MBSs comb can be effectively modulated by the quantum dots. For only one “QD-MBSs-QD” building block, N = 1, the current noise cross correlation is negative at small bias and becomes positive at lager bias. A zero bias conductance peak of 2e2/h can be found, and it splits with ϵu increasing, where ϵu is the energy level of up-part QD. For N ≥ 2, the current noise cross correlation is zero at small bias and becomes positive at larger bias. Different from the case of N = 1, the zero bias conductance peak is 4e2/h due to the local Andreev reflection. Besides, we find that the differential conductance is robust against the influence of N.

Photocurrent measurements in topological insulator Bi2Se3 nanowires

Circular photogalvanic currents are a promising approach for spin-optoelectronics. To date, such currents have been induced in topological insulator flakes or extended films. It is not clear whether they can be generated in nanodevices. In this paper, we demonstrate the generation of circular photogalvanic currents in Bi2Se3 nanowires. Each nanowire shows topological surface states. Here, we generate and distinguish the different photocurrent contributions via the driving light wave. We separate the circular photogalvanic currents from those due to thermal Seebeck effects through controlling laser light polarization. The results reveal a spin-polarized surface-Dirac electron flow in the nanowires arising from spin-momentum locking and spin–orbit effects. The second photocurrent contribution described in this Letter is caused by the thermal Seebeck effect. By scanning the photocurrent, it can be spatially resolved; upon reversing the gradient direction along the nanowire, the photocurrent changes its sign, and close to the gold contacts, the amplitudes of the different photocurrent contributions are affected by the proximity to the contacts. In the center of the nanowires, where the effects from the gold contact/topological insulator stacks vanish, the spin-polarized current remains constant along the nanowires. This allows the all-optical spin current generation in topological insulator nanowires and hybrid structures on the nanoscale, one goal of spin-orbitronics.

Majorana Bound State Leakage to Impurity in Su-Schrieffer-Heeger-Rashba Scenario

We show anomalous features of Majorana Bound State leakage in the situation where topological Rashba nanowire is dimerized according to the Su-Schrieffer-Heeger (SSH) scenario and an impurity is present at one of the ends of the system. We find that two topological branches: usual, indigenous to Rashba nanowire and dimerized one, existing as a result of SSH dimerization of nanowire, have different asymmetry of spin polarization that can be explained by opposite order of bands taking part in topological transitions. Additionally, introduction of an impurity to the dimerized nanowire influences the leakage of Majorana bound states into the trivial impurity, due to the emergence of Andreev bound states that behave differently whether the system is or is not in topological phase. This results in the pinning of zero energy states to the impurity site for some range of parameters.

The properties of Majorana bound states in topological superconductor nanowire

We investigate the differential conductance in topological superconductor nanowire. The appropriate proximity-induced s-wave superconductivity and an externally applied magnetic field can derive a critical value to illustrate the topological state of the nanowire. When the strength of magnetic field is below the critical value, the nanowire produces an interior gap, and the interior gap decreases as the magnetic field increases. When the magnetic field is bigger than the critical value, the interior gap disappears, and zero-bias conductance peak appears which indicates that nanowire transits into the topological phase. In particular, with increasing of the chemical potential, the nanowire is changed into the topological phase with more stronger magnetic field applied. As long as the spin-orbit coupling and magnetic field axes are perpendicular, the nanowire can always be transited into the topological phase at a sufficiently large magnetic field.

Electric Control of Fermi Arc Spin Transport in Individual Topological Semimetal Nanowires.

The exotic topological surface states of Dirac or Weyl semimetals, namely Fermi arcs, are predicted to be spin polarized, while their spin polarization nature is still not revealed by transport measurements. Here, we report the spin-polarized transport in a Dirac semimetal Cd_{3}As_{2} nanowire employing the ferromagnetic electrodes for spin detection. The spin-up and spin-down states can be changed by reversing the current polarity, showing the spin-momentum locking property. Moreover, the nonlocal measurements show a high fidelity of the spin signals, indicating the topological protection nature of the spin transport. As tuning the Fermi level away from the Dirac point by gate voltages, the spin signals gradually decrease and finally are turned off, which is consistent with the fact that the Fermi arc surface state has the maximum ratio near the Dirac point and disappears above the Lifshitz transition point. Our results should be valuable for revealing the transport properties of the spin-polarized Fermi arc surface states in topological semimetals.