Conductance Peak Research Articles

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

Structure Identification for Force-Induced Reaction Using Single-Molecule Conductance Measurement

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.

Large zero bias peaks and dips in a four-terminal thin InAs-Al nanowire device

We report electron transport studies of a thin InAs-Al hybrid semiconductor-superconductor nanowire device using a four-terminal design. Compared to previous works, thinner InAs nanowire (diameter less than 40 nm) is expected to reach fewer sub-band regime. The four-terminal device design excludes electrode contact resistance, an unknown value which has inevitably affected previously reported device conductance. Using tunneling spectroscopy, we find large zero-bias peaks (ZBPs) in differential conductance on the order of $2e^2/h$. Investigating the ZBP evolution by sweeping various gate voltages and magnetic field, we find a transition between a zero-bias peak and a zero-bias dip while the zero-bias conductance sticks close to $2e^2/h$. We discuss a topologically trivial interpretation involving disorder, smooth potential variation and quasi-Majorana zero modes.

Random matrix theory for the robustness, quantization, and end-to-end correlation of zero-bias conductance peaks in a class D ensemble

We develop a general theory to study strong random quenched disorder effects in systems of experimental relevance in the search for Majorana zero modes in topological superconductors. Using the random matrix theory in a class D ensemble, we simulate the transport properties of random quantum dots by attaching leads, and calculating the differential conductance in the $S$-matrix formalism. To add the concept of the length to the random system so that disordered Majorana nanowires can be simulated by the random matrix theory, we generalize the model of a single quantum dot to a chain of quantum dots by analogy with the superconductor-semiconductor nanowire Majorana platform. We first define a new concept, the robustness of zero-bias conductance peaks, in terms of an effective random Hamiltonian considering the self-energy of leads. We then study the joint distribution for the robustness and zero-bias conductance peaks, and find a strong correlation that the zero-bias conductance peak with stronger robustness is also prone to carry a larger conductance peak near $2{e}^{2}/h$. This trend is more prominent in shorter chains than in longer chains. This is consistent with experimentally observed zero-bias conductance associated with disorder-induced trivial Andreev bound states (the so-called ugly zero-bias peaks). Finally, we study the end-to-end correlation of the disorder-induced zero-bias conductances from two leads by calculating the normalized mutual information, which estimates the degrees of the correlation arising from the trivial zero-bias conductance peaks. Our work provides an estimate of several important metrics used in superconductor-semiconductor experiments to determine the nature of zero-bias conductance peaks, including the robustness, the quantization, and the end-to-end correlation of the trivial zero-bias peaks. Therefore, in order to claim any evidence for the Majorana zero modes, one must establish the observed zero-bias conductance peaks to have considerable statistical significance well beyond what we find in this work to exist for the trivial peaks.

Quality factor for zero-bias conductance peaks in Majorana nanowire

Despite recent experimental progress towards observing large zero-bias conductance peaks (ZBCPs) as signatures of Majorana modes, confusion remains about whether Majorana modes have been observed. This is in part due to the theoretical prediction of fine-tuned trivial (i.e., non-topological) zero-bias peaks that occur because of uncontrolled quantum dots or disorder potentials. While many aspects of the topological phase can be somewhat fine-tuned because the topological phase space is often small, the quantized height of the ZBCP associated with a Majorana mode is known to be robust at sufficiently low temperatures even as the tunnel barrier is pinched off to vanishingly small normal-state conductance. The key shortcoming of the existing experimental works is an acute lack of stability of the putative Majorana mode features, indicating the probable absence of a topological phase, and the current paper suggests specific experimentally accessible measures for a careful quantitative analysis of the measured ZBCP stability. In this paper, we study how the counter-intuitive robustness of the ZBCP height to the tunnel barrier strength can be used to distinguish Majorana modes from non-topological ZBCPs. To this end, we introduce a dimensionless quality factor $F$ to quantify the robustness of the ZBCP height based on the range of normal-state (i.e. above-gap) conductance (which depends crucially on the tunnel barrier height) over which the ZBCP height remains within a pre-specified range of quantization. By computing this quality factor $F$ together with the topological characteristics for a wide range of models and parameters, we find that Majoranas are significantly more robust (i.e., have a higher value of $F$) compared with non-topological ZBCPs in the ideal low-temperature limit. Even at a temperature as high as the experimentally used $20$ mK, we find that we can set a threshold value of $F\sim 2.5$ (for $\epsilon=0.1$) so that ZBCPs associated with a quality factor $F>2.5$ are likely topological and $F\ll 2.5$ are topologically trivial. More precisely, the value of $F$ is operationally related to the degree of separation of the Majorana modes in the system, although $F$ uses only the experimentally measured tunnel conductance properties. Finally, we discuss how the quality factor $F$ measured in a transport setup can help estimate the quality of topological qubits made from Majorana modes. In particular, we show that if the induced gap can be enhanced somehow beyond the currently available $\sim 30$ $\mu$eV in InAs/Al samples, large (small) values of $F$ could easily distinguish between stable topological (unstable trivial) ZBCPs with the quantum dot induced quasi-Majorana bound states occasionally behaving similar to topological Majorana modes in short wires.

Self-consistent theory of current injection into d and d + is superconductors

We present results for the steady-state nonlinear response of a superconducting film connected to normal-metal reservoirs under voltage bias, allowing for a subdominant s-wave component appearing near the interfaces. Our investigation is based on a current-conserving theory that self-consistently includes the non-equilibrium distribution functions, charge imbalance, and the voltage-dependencies of order parameters and scalar impurity self-energies. For a pure d-wave superconductor with [110] orientation of the interfaces to the contacts, the conductance contains a zero-bias peak reflecting the large density of zero-energy interface Andreev bound states. Including a subdominant s-wave pairing channel, it is in equilibrium energetically favorable for an s-wave order parameter component to appear near the interfaces in the time-reversal symmetry breaking combination d + is. The Andreev states then shift to finite energies in the density of states. Under voltage bias, we find that the non-equilibrium distribution in the contact area causes a rapid suppression of the s-wave component to zero as the voltage . The resulting spectral rearrangements and voltage-dependent scattering amplitudes lead to a pronounced non-thermally broadened split of the zero-bias conductance peak that is not seen in a non-selfconsistent Landauer–Büttiker scattering approach.

Superconducting Proximity in Intrinsic Magnetic Topological Insulator MnBi2Te4-NbN Hybrid Device Modulated by Coulomb Blockade Effect.

The combination of nontrivial topology, magnetism, and superconductivity could offer the potential to realize exotic excitations of quasiparticles. MnBi2Te4, as an intrinsic magnetic topological insulator, may be a good platform to create Majorana fermions if coupled to an s-wave superconductor. Here, we report the transport properties of a MnBi2Te4-NbN hybrid device. This device exhibits clear Coulomb blockade oscillations. We observe a large zero-bias conductance peak that exists over considerable changes in gate voltage, magnetic field, and temperature, which is interpreted as a not fully developed supercurrent. The zero-bias peak shows a nonmonotonic evolution with a magnetic field and an abrupt π phase shift with changing temperature. Zero-energy bound states and a topological phase transition may exist in this hybrid system. Our results provide the first experimental investigation into the properties of the intrinsic magnetic topological insulator/superconductor hybrid structures modulated by the Coulomb blockade effect.

First-principles calculations of electrical conductivities of edge-modified graphene nanoribbons: Strain effect

We investigate the influence of strain on the electrical properties of graphene nanoribbons that have potential applications in making sensors and other optoelectronic devices. In particular, we chose pristine armchair graphene nanoribbons with 7 zigzag edges (7aGNRsH), boron doped armchair graphene nanoribbons with 7 zigzag edges (7aGNRsH-B) and armchair graphene nanoribbons with 7 zigzag edges that have one carbon atom vacancy (7aGNRsH-V). Based on first-principles calculations, results show that pristine unstrained 7aGNRsH is electrically nonconductive but turns to be electrically conductive in a wide range of energy spectrum , e.g., from IR to visible to UV, due to the application of strain engineering. In metallic unstrained and strained 7aGNRsH-B and 7aGNRsH-V, non-vanishing electrical conductivity in the IR, visible and UV energy spectrum regimes are observed. We also investigate the influence of strain on the Berry curvature of 7aGNRsH, 7aGNRsH-B and 7aGNRsH-V nanoribbons. The results show that fermions are spread through out the Brillouin zone in the reciprocal space for semiconducting unstrained 7aGNRsH but localized near the Γ -point for strained 7aGNRsH that have out-of-plane deformations due to strain engineering. For metallics 7aGNRsH-B and 7aGNRsH-V, Berry curvature plots show that fermions are localized far away from the Γ -point. In two atom boron doped p-type armchair graphene nanoribbons with 7 zigzag edges (7aGNRsH-2B), large peaks in electrical conductivity at IR energy spectrum regimes can be observed. These peaks of electrical conductivities in 7aGNRSH-2B may be detectable in experimentally synthesized structure in Cloke et al. (2015) [1] . • In Figs. 2 , 3 , 4 , we have provided results for band structures calculations obtained from pw.x and maximally localized wannier 90 method that are in excellent agreement. This provides foundation for investigation of influence of strain on the Berry curvature and electrical conductivity. Note that 7 boron doped armchair graphene nanoribbons in Fig. 3 and one carbon vacancy in 7 armchair graphene nanoribbons in Fig. 4 are metallic in nature. • When we apply strain, we have a relaxed shape armchair graphene nanoribbon and thus we expect localization of fermions near Gamma-point which can be clearly seen in Fig. 8 . • Since Fermi energy stays below the top of the valence band in metallic boron doped and vacancy of one carbon atom in graphene nanoribbon, we see the cut-off point in Berry curvature in Fig. 5 . • Most importantly in Figs. 6 , 11 and 12 , we see that intrinsic 7 armchair graphene nanoribbon is not conductive but metallic boron doped and vacancy created nanoribbon are conductive in the wide range of energy spectrum regimes, e.g., from IR to visible to UV.

Effect of Virtual Reality Hypnosis on Pain Threshold and Neurophysiological and Autonomic Biomarkers in Healthy Volunteers: Prospective Randomized Crossover Study.

BackgroundVirtual reality hypnosis (VRH) is a promising tool to reduce pain. However, the benefits of VRH on pain perception and on the physiological expression of pain require further investigation.ObjectiveIn this study, we characterized the effects of VRH on the heat pain threshold among adult healthy volunteers while monitoring several physiological and autonomic functions.MethodsSixty healthy volunteers were prospectively included to receive nociceptive stimulations. The first set of thermal stimuli consisted of 20 stimulations at 60°C (duration 500 milliseconds) to trigger contact heat evoked potentials (CHEPs). The second set of thermal stimuli consisted of ramps (1°C/second) to determine the heat pain threshold of the participants. Electrocardiogram, skin conductance responses, respiration rate, as well as the analgesia nociception index were also recorded throughout the experiment.ResultsData from 58 participants were analyzed. There was a small but significant increase in pain threshold in VRH (50.19°C, SD 1.98°C) compared to that in the control condition (mean 49.45°C, SD 1.87; P<.001, Wilcoxon matched-pairs signed-rank test; Cohen d=0.38). No significant effect of VRH on CHEPs and heart rate variability parameters was observed (all P>0.5; n=22 and n=52, respectively). During VRH, participants exhibited a clear reduction in their autonomic sympathetic tone, as shown by the lower number of nonspecific skin conductance peak responses (P<.001, two-way analysis of variance; n=39) and by an increase in the analgesia nociception index (P<.001, paired t-test; n=40).ConclusionsThe results obtained in this study support the idea that VRH administration is effective at increasing heat pain thresholds and impacts autonomic functions among healthy volunteers. As a nonpharmacological intervention, VRH has beneficial action on acute experimental heat pain. This beneficial action will need to be evaluated for the treatment of other types of pain, including chronic pain.

Distinguishing Majorana and Andreev bound states in a topological superconducting nanowire with a potential barrier

We propose a scheme to distinguish the Majorana bound states (MBSs) and the trivial Andreev bound states (ABSs) in a topological superconducting nanowire by applying a sharp local potential barrier on one part of the wire. The position and strength of the local potential barrier could be modulated by a gate voltage applied to the nanowire. It is found that a new pair of MBSs appear at both sides of the potential in the presence of the local potential barrier, as a manifestation of the topological nature. Differently, the local potential barrier does not induce extra ABSs. Such a difference can be traced back to the evolution of the conductance spectrum with the Zeeman field or the local potential strength. When modulating the local potential strength, the zero-bias conductance peak induced by MBSs can be split into two-, three-, or four-peak structures, while the zero-bias peak by ABSs is only split into two-peak form.

Engineering topological states in atom-based semiconductor quantum dots

The realization of controllable fermionic quantum systems via quantum simulation is instrumental for exploring many of the most intriguing effects in condensed-matter physics1–3. Semiconductor quantum dots are particularly promising for quantum simulation as they can be engineered to achieve strong quantum correlations. However, although simulation of the Fermi–Hubbard model4 and Nagaoka ferromagnetism5 have been reported before, the simplest one-dimensional model of strongly correlated topological matter, the many-body Su–Schrieffer–Heeger (SSH) model6–11, has so far remained elusive—mostly owing to the challenge of precisely engineering long-range interactions between electrons to reproduce the chosen Hamiltonian. Here we show that for precision-placed atoms in silicon with strong Coulomb confinement, we can engineer a minimum of six all-epitaxial in-plane gates to tune the energy levels across a linear array of ten quantum dots to realize both the trivial and the topological phases of the many-body SSH model. The strong on-site energies (about 25 millielectronvolts) and the ability to engineer gates with subnanometre precision in a unique staggered design allow us to tune the ratio between intercell and intracell electron transport to observe clear signatures of a topological phase with two conductance peaks at quarter-filling, compared with the ten conductance peaks of the trivial phase. The demonstration of the SSH model in a fermionic system isomorphic to qubits showcases our highly controllable quantum system and its usefulness for future simulations of strongly interacting electrons.

Electronic properties and quantum transport in functionalized graphene Sierpinski-carpet fractals

Recent progress in the controllable functionalization of graphene surfaces enables the experimental realization of complex functionalized graphene nanostructures, such as Sierpinski carpet (SC) fractals. Herein, we model the SC fractals formed by hydrogen and fluorine functionalized patterns on graphene surfaces, namely, H-SC and F-SC, respectively. We then reveal their electronic properties and quantum transport features. From the calculated results of the total and local densities of state, we find that states in H-SC and F-SC have two characteristics: (i) low-energy states inside about $|E/t|\ensuremath{\le}1$ (with $t$ as the nearest-neighbor hopping) are localized inside free graphene regions due to the insulating properties of functionalized graphene regions and (ii) high-energy states in F-SC have two special energy ranges including $\ensuremath{-}2.3&lt;E/t&lt;\ensuremath{-}1.9$ with localized holes only inside free graphene areas and $3&lt;E/t&lt;3.7$ with localized electrons only inside fluorinated graphene areas. The two characteristics are further verified by the real-space distributions of normalized probability density. We analyze the fractal dimension of their quantum conductance spectra and find that conductance fluctuations in these structures follow the Hausdorff dimension. We calculate their optical conductivity and find that several additional conductivity peaks appear in high-energy ranges due to the adsorbed H or F atoms.

Screening the habitat function of biochar-amended vineyard soils at field plot-scale, based on invertebrate avoidance behaviour

There is an urge for rapid, cost-effective and ecologically representative tools that inform on possible changes in habitat function of biochar-amended soils, at representative spatial and temporal scales. We employed a battery of invertebrate avoidance behaviour tests to screen and biomonitor a vineyard soil amended with a premium grade mixed-wood chip biochar (applied alone or mixed with compost) at plot–scale, at 4 or 40 t ha−1, over 12 months. The assays combined representative organisms with complementary ecological functions in agroecosystems, i.e. earthworms (Eisenia andrei), collembolans (Folsomia candida) and isopods (Porcellionides pruinosus). In all treatments, soil habitat function was within the recommended limits for the selected invertebrate groups, by the corresponding soil quality guideline or protocol. However, collembolans exhibited significant avoidance behaviour from 6-month aged biochar treatments, coinciding with a strong peak in soil electrical conductivity (EC), suggestive of osmotic stress. Earthworms responded less sensitively than collembolans, with isopods being the least sensitive group to biochar amendments. Further, there was a preference by isopods for freshly-amended soil, whereas earthworms and collembolans preferred 12-month aged biochar amended soil, both of which can be explained, at least partly, by nutrient inputs. Overall, results show that invertebrate avoidance behaviour, based on multiple test groups, can be used to screen and complement site-specific biochar risk assessment strategies, for early decision-making and management. For screening batteries of mixed wood biochars, this study further supports the combined use of collembolans and earthworms, with isopods as a complementary test group.

Many-Body Study of Iron(III)-Bound Human Serum Transferrin.

We present the very first density functional theory and dynamical mean field theory calculations of iron-bound human serum transferrin. Peaks in the optical conductivity at 250, 300, and 450 nm were observed, in line with experimental measurements. Spin multiplet analysis suggests that the ground state is a mixed state with high entropy, indicating the importance of strong electronic correlation in this system’s chemistry.

Characterization of Interface and Bulk Traps in Ultrathin Atomic Layer-Deposited Oxide Semiconductor MOS Capacitors With HfO2/In2O3 Gate Stack by C-V and Conductance Method

Oxide semiconductors have attracted revived interest for complementary metal–oxide–semiconductor (CMOS) back-end-of-line (BEOL) compatible devices for monolithic 3-dimensional (3D) integration. To obtain a high-quality oxide/semiconductor interface and bulk semiconductor, it is critical to enhance the performance of oxide semiconductor transistors. Atomic layer-deposited (ALD) indium oxide (In2O3) has been reported with superior performance such as high drive current, high mobility, steep subthreshold slope, and ultrathin channel. In this work, the interface and bulk traps in the MOS gate stack of ALD In2O3 transistors are systematically studied by using the C–V and conductance method. A low EOT of 0.93 nm is achieved directly from the accumulation capacitance in C–V measurement, indicating a high-quality gate oxide and oxide/semiconductor interface. Defects in bulk In2O3 with energy levels in the subgap are confirmed to be responsible for the conductance peak in GP/ω versus ω curves by TCAD simulation of C–V and G–V characteristics. A high n-type doping of 1×1020/cm3 is extracted from C–V measurement. A high subgap density of states (DOS) of 3.3×1020 cm−3 eV−1 is achieved using the conductance method, which contributes to the high n-type doping and high electron density. The high n-type doping further confirms the capability of channel thickness scaling because the charge neutrality level aligns deeply inside the conduction band.

Origin of photo-enhanced hysteretic electrical conductance in nanostructured silicon-based heterojunction

Photo-enhanced hysteretic I–V curves have been observed under reverse bias in a p-i-n structure containing electrochemically etched nanostructured silicon (Si) sandwiched between p-Si and n-type a-Si:H layers. These curves have been found to depend on intensity of incident illumination and structural morphology of the nanostructured Si layer. The conductance in trace path is lower than that in retrace path. Charge transport mechanism in this structure has been interpreted using microscopic description of charge trapping and detrapping in the defect states present at the interface of nanocrystalline silicon core and oxide shell in the active layer. An applied voltage dependent probability distribution of trapping and detrapping has been calculated in light of classical random walk problem. The trapping/detrapping of charges leading to development/destruction of potential barriers in the path of charge flow shows an analogy with the river bed deposition/erosion. The rate of trapping has been considered to depend on the empty defect states whereas the rate of detrapping depends on the already filled defects. Moreover, the rate of both trapping and detrapping is expected to depend on the charge flow rate. All these considerations lead the I–V relations for trace and retrace paths in reverse bias fitting nicely with experimental I–V loops. The observed peaks in the voltage dependent dynamic conductance in trace and retrace paths have been explained as a consequence of development and destruction of two barriers in the active layer for electrons and holes separately. Best fit values of the fitting parameters indicates that the trace path is dominated by holes whereas the retrace path is dominated by electronic transport. The difference in mobility of electron and hole leads to different trapping and detrapping rates in the two paths resulting in the observed hysteresis.

Quantum transport in p-type narrow channel with DC-biased double finger gate

The quantum transport in a p-type narrow channel device with a DC-biased double finger gate is investigated using a Luttinger four-band Hamiltonian and a propagation matrix approach. The results show that when the potential energy modulation effect produced by the finger gate is large, the quantum transport shows a resonance behavior indicative of large barriers in a double-barrier system. A simple analytical method based on an ideal resonant square quantum well approximation is used to predict the energies of the resonance peaks contributed by the light and heavy holes. The results show that the first conductance peak is formed at 2e2/h and is contributed by the heavy holes. A second conductance resonance peak is formed at 3e2/h due to the semi-reflection of the heavy holes and resonant transmission of the light holes. Finally, a third conductance resonance peak is formed at 4e2/h as a result of the concurrent resonant transmission of the light holes and heavy holes.

Valley filtering in graphene under a magnetic proximity

We investigate valley-dependent electronic transport properties of graphene in close proximity to a thick ferromagnetic insulator (FI). It is found that spin-resolved valley filtering can be achieved under the interplay between the stray field emanated from the FI and the exchange-proximity interaction. In the intraband-tunneling regime, there exists a window of Fermi energy where both the spin and valley polarization of the output current are almost complete $(\ensuremath{\approx}\ensuremath{-}100%)$. In the interband-tunneling regime, the valley polarization can approach $100%$ or $\ensuremath{-}70%$ near some conductance peaks, while the spin polarization is near $100%$ in a wide window of Fermi energy. The role of valley-Zeeman coupling is examined for both cases. The spin-valley filtering is found to be tunable by electric gating. Our findings are helpful for designing a spin-valley filter based on graphene with imprinted ferromagnetism.

Destructive quantum interference phenomenon in series-coupled double quantum dots

In this work, the transport of a model consists of two quantum dots in series and Luttinger liquid leads is researched. The motivation is to reveal the interference characteristics. We comprehensively study the influence of the intralead Coulomb interaction, temperature, detuning levels of the dots, and inter-dot tunneling on transport. By applying the nonequilibrium Green’s function method, a general current formula is derived. The existence of the intralead interaction leads to the appearance of a destructive quantum interference anti-resonant dip at the zero-bias voltage of the differential conductance. The gate-tunable conductance peaks or dips depending on the constructive or destructive interference are demonstrated when the energy levels of the two dots are aligned. While when the energy levels are misaligned, the intriguing Fano anti-resonant interference phenomenon occurs. These results offer a new approach for designing quantum coherent devices.

On-demand large conductance in trivial zero-bias tunneling peaks in Majorana nanowires

Motivated by recent experiments that report the almost-generic large-conductance peaks without very extensive fine-tuning, we propose an alternative mechanism through direct theoretical simulations that can explain the large zero-bias conductance peaks being generated on-demand in the nontopological regime in Majorana nanowires by satisfying the following three sufficient conditions: (i) strong potential disorder in the bulk of the nanowire, suppressing the topological regime; (ii) strong suppression of the disorder near the nanowire ends connecting to the tunneling leads, perhaps because of screening by the metallic leads and gates; and (iii) low tunnel barrier strength leading to large tunneling amplitude. The third condition is typically achieved experimentally by fine-tuning the tunnel barrier and the first condition is generic in all existing nanowires by virtue of considerable sample disorder induced by unintentional random quenched charged impurities. The second condition is likely to apply to many samples since the disorder potential would be typically screened more strongly at the wire ends because of the large metallic tunnel pads used experimentally. We show that the resultant tunneling conductance manifests large trivial zero-bias peaks almost on demand, and such peaks could be $\sim 2e^2/h$, when appropriately fine-tuned by the tunnel barrier strength and the temperature, as reported experimentally. Our work not only solves the mystery in recent experiments that the observations of the large zero-bias conductance peaks are generic by proposing a theoretically possible mechanism but also explains why these hypothesized conditions are naturally satisfied in experiments.