Conductance Dip Research Articles

Theoretical valley-polarized subgap transport and intravalley pairing states in a silicene-based antiferromagnet–superconductor junction

We theoretically study the valley-polarized subgap transport and intravalley pairing states in silicene-based antiferromagnet/superconductor (AF/SC) junctions. It is found that in the absence of an electric field, the antiferromagnetic order induced in silicene can give rise to valley-polarized states that strongly affect the subgap conductance. With the increasing antiferromagnetic exchange field, the gap-edge Andreev-resonant peak is replaced by broadened features for the homo-SC model whereas by a sharp conductance dip for the bulk-SC one. This significant difference arises from the intravalley Andreev reflection caused by the valley-mixing scattering in the bulk-SC model, which can be enhanced by the antiferromagnetic order. Particularly, this intravalley pairing process can be switched on or off by adjusting the spin polarization through the electric field applied in the AF region. Our findings not only pave a new road to employ antiferromagnetic materials in valleytronics, but also facilitate the verification and detection of potential intravalley pairing state and valley polarization in silicene.

Majorana bound state in the continuum and Fano effect formed by the coupling of two Andreev-reflection channels

Majorana-induced Andreev reflection (AR) is theoretically investigated by considering the interplay between two AR channels, each of which is formed by the lead and its coupled topological-superconducting nanowire with Majorana bound states. By means of the Scattering matrix and nonequilibirum Green’s function technique, relevant observables are analyzed, including the differential conductance, the density of states, and shot-noise Fano factors. It is found that Majorana bound states in the continuum (MBIC) can be constructed, under the condition of two uniform nanowires and inter-channel couplings. Otherwise, the Fano effect is allowed to occur, which is manifested as the appearance of additional conductance dips. In particular, the Fano antiresonance can be induced by constructing the identical inter-channel couplings. All these results can be helpful in understanding the MBIC contributed by the Majorana-assisted ARs.

Majorana tunneling in a one-dimensional wire with non-Hermitian double quantum dots

The combination of non-Hermitian physics and Majorana fermions can give rise to new effects in quantum transport systems. In this work, we investigate the interplay of PT-symmetric complex potentials, Majorana tunneling and interdot tunneling in a non-Hermitian double quantum dots system. It is found that in the weak-coupling regime the Majorana tunneling has pronounced effects on the transport properties of such a system, manifested as splitting of the single peak into three and a reduced 1/4 peak in the transmission function. In the presence of the PT-symmetric complex potentials and interdot tunneling, the 1/4 central peak is robust against them, while the two side peaks are tuned by them. The interdot tunneling only induces asymmetry, instead of moving the conductance peak, due to the robustness of the Majorana modes. There is an exceptional point induced by the union of Majorana tunneling and interdot tunneling. With increased PT-symmetric complex potentials, the two side peaks will move towards each other. When the exceptional point is passed through, these two side peaks will disappear. In the strong-coupling regime, the Majorana fermion induces a 1/4 conductance dip instead of the three-peak structure. PT-symmetric complex potentials induce two conductance dips pinned at the exceptional point. These effects should be accessible in experiments.

Holographic superfluid with gauge–axion coupling

We have constructed a holographic superfluid with gauge–axion coupling. Depending on whether the coupling is positive or negative, the system displays metallic or insulating behavior in its normal state. A significant feature of the system is the appearance of a mid-IR peak in the alternating current (AC) conductivity in a certain range of parameters. This peak arises due to competition between explicit symmetry breaking (ESB) and spontaneous symmetry breaking (SSB), which results in the presence of a pseudo-Goldstone mode. Moreover, a dip in low-frequency AC conductivity is observed, stemming from the excitation of the SSB Goldstone mode. In the superfluid phase, the effect of gauge–axion coupling on the condensation or superfluid energy gap is only amplified in the presence of strong momentum dissipation. Notably, for the case with negative gauge–axion coupling, a hard-gap-like behavior at low frequency and a pronounced peak at intermediate frequency are observed, indicating that the evolution of the superfluid component is distinct from that of positive coupling.

Bytizite Cu3SbSe3: Solid-State Synthesis and Thermoelectric Performance

Bytizite (Cu3SbSe3 ) has attracted interest as a promising thermoelectric material because of its ultralow thermal conductivity; however, there are few experimental studies. This study investigated the optimal processing conditions for the synthesis of Cu3SbSe3 using mechanical alloying (MA) and hot pressing (HP). The MA powder exhibited an orthorhombic Cu3SbSe3 phase, which remained even after HP. However, secondary phases of permingeatite (Cu3SbSe4) and berzelianite (Cu1.78Se) were also identified in the X-ray diffraction patterns. Thermal analysis revealed that the MA powder and HP compacts exhibited a large endothermic peak near 727 K, which corresponds to the melting point of Cu3SbSe3 . Dense compacts with a relative density higher than 99% were obtained at HP temperatures above 573 K. Microstructural and elemental analyses confirmed the presence of the secondary phase Cu3 SbSe4 in the matrix of Cu3SbSe3 . However, the Cu1.78Se phase could not be observed. All specimens exhibited an electrical conductivity of (0.66–1.06) × 10 3 Sm-1, a Seebeck coefficient of 324–376 µVK-1, and a power factor of 0.09–0.11 mWm-1K-2 at 623 K. The thermal conductivity was lower than 0.7 Wm-1K-1 in the measured temperature range, mainly due to the phonon scattering caused by the lone-pair electrons of Sb. A dip in thermal conductivity was observed at 423 K, which was possibly caused by the order-disorder transition of bytizite. The dimensionless figure of merit ZT increased with increasing temperature, and the maximum <i>ZT</i> was 0.16 at 623 K.

Gate-Tunable Negative Differential Conductance in Hybrid Semiconductor–Superconductor Devices

Negative differential conductance (NDC) serves as a crucial characteristic that reveals various underlying physics and transport process in hybrid superconducting devices. We report the observation of gate-tunable NDC outside the superconducting energy gap on two types of hybrid semiconductor–superconductor devices, i.e., normal metal–superconducting nanowire–normal metal and normal metal–superconducting nanowire–superconductor devices. Specifically, we study the dependence of the NDCs on back-gate voltage and magnetic field. When the back-gate voltage decreases, these NDCs weaken and evolve into positive differential conductance dips; and meanwhile they move away from the superconducting gap towards high bias voltage, and disappear eventually. In addition, with the increase of magnetic field, the NDCs/dips follow the evolution of the superconducting gap, and disappear when the gap closes. We interpret these observations and reach a good agreement by combining the Blonder–Tinkham–Klapwijk (BTK) model and the critical supercurrent effect in the nanowire, which we call the BTK-supercurrent model. Our results provide an in-depth understanding of the tunneling transport in hybrid semiconductor–superconductor devices.

Hybridization-Controlled Pseudogap State in the Quantum Critical Superconductor CeCoIn_{5}.

The origin of the partial suppression of the electronic density states in the enigmatic pseudogap behavior, which is at the core of understanding high-T_{c} superconductivity, has been hotly contested as either a hallmark of preformed Cooper pairs or an incipient order of competing interactions nearby. Here, we report the quasiparticle scattering spectroscopy of the quantum critical superconductor CeCoIn_{5}, where a pseudogap with energy Δ_{g} was manifested as a dip in the differential conductance (dI/dV) below the characteristic temperature of T_{g}. When subjected to external pressure, T_{g} and Δ_{g} gradually increase, following the trend of increase in quantum entangled hybridization between the Ce 4f moment and conduction electrons. On the other hand, the superconducting (SC) energy gap and its phase transition temperature shows a maximum, revealing a dome shape under pressure. The disparate dependence on pressure between the two quantum states shows that the pseudogap is less likely involved in the formation of SC Cooper pairs, but rather is controlled by Kondo hybridization, indicating that a novel type of pseudogap is realized in CeCoIn_{5}.

Merging Nanowires and Formation Dynamics of Bottom‐Up Grown InSb Nanoflakes

AbstractIndium Antimonide (InSb) is a semiconductor material with unique properties, that are suitable for studying new quantum phenomena in hybrid semiconductor‐superconductor devices. The realization of such devices with defect‐free InSb thin films is challenging, since InSb has a large lattice mismatch with most common insulating substrates. Here, the controlled synthesis of free‐standing 2D InSb nanostructures, termed as “nanoflakes”, on a highly mismatched substrate is presented. The nanoflakes originate from the merging of pairs of InSb nanowires grown in V‐groove incisions, each from a slanted and opposing {111}B facet. The relative orientation of the two nanowires within a pair, governs the nanoflake morphologies, exhibiting three distinct ones related to different grain boundary arrangements: no boundary (type‐I), Σ3‐ (type‐II), and Σ9‐boundary (type‐III). Low‐temperature transport measurements indicate that type‐III nanoflakes are of a relatively lower quality compared to type‐I and type‐II, based on field‐effect mobility. Moreover, type‐III nanoflakes exhibit a conductance dip attributed to an energy barrier pertaining to the Σ9‐boundary. Type‐I and type‐II nanoflakes exhibit promising transport properties, suitable for quantum devices. This platform hosting nanoflakes next to nanowires and nanowire networks can be used to selectively deposit the superconductor by inter‐shadowing, yielding InSb‐superconductor hybrid devices with minimal post‐fabrication steps.

Mining association rules between the granulation feasibility and physicochemical properties of aqueous extracts from Chinese herbal medicine in fluidized bed granulation.

Fluidized bed granulation (FBG) is a widely used granulation technology in the pharmaceutical industry. However, defluidization caused by the formation of large aggregates poses a challenge to FBG, particularly in traditional Chinese medicine (TCM) due to its complex physicochemical properties of aqueous extracts. Therefore, this study aims to identify the complex relationships between physicochemical characteristics and defluidization using data mining methods. Initially, 50 types of TCM were decocted and assessed for their potential influence on defluidization using a set of 11 physical properties and 10 chemical components, utilizing the loss rate as an evaluation index. Subsequently, the random forest (RF) and Apriori algorithms were utilized to uncover intricate association rules among physicochemical characteristics and defluidization. The RF algorithm analysis revealed the top 8 critical factors associated with defluidization. These factors include physical properties like glass transition temperature (Tg) and dynamic surface tension (DST) of DST100ms, DST1000ms, DST10ms and conductivity, in addition to chemical components such as fructose, glucose and protein contents. The results from Apriori algorithm demonstrated that lower Tg and conductivity were associated with an increased risk of defluidization, resulting in a higher loss rate. Moreover, DST100ms, DST1000ms and DST10ms exhibited a contrasting trend in the physical properties Specifically, defluidization probability increases when Tg and conductivity dip below 29.04℃ and 6.21 ms/m respectively, coupled with DST10ms, DST100ms and DST1000ms values exceeding 70.40 mN/m, 66.66 mN/m and 61.58 mN/m, respectively. Moreover, an elevated content of low molecular weight saccharides was associated with a higher occurrence of defluidization, accompanied by an increased loss rate. In contrast, protein content displayed an opposite trend regarding chemical properties. Precisely, the defluidization likelihood amplifies when fructose and glucose contents surpass 20.35 mg/g and 34.05 mg/g respectively, and protein concentration is less than 1.63 mg/g. Finally, evaluation criteria for defluidization were proposed based on these results, which could be used to avoid this situation during the granulation process. This study demonstrated that the RF and Apriori algorithms are effective data mining methods capable of uncovering key factors affecting defluidization.

Spin and valley polarization of an MoS2 zigzag nanoribbon with a magnetic barrier via Fano resonance

For monolayer ${\mathrm{MoS}}_{2}$, a magnetic field causes the degeneracy of the $K$ and ${K}^{\ensuremath{'}}$ valleys to be lifted, owing to the valley Zeeman effect. Thus, in a monolayer ${\mathrm{MoS}}_{2}$ nanoribbon system with a magnetic barrier, electrons in these valleys exhibit different conductance behaviors when passing through the barrier. One valley shows a strong dip in conductance caused by Fano resonance, i.e., quantum interference between continuum states and Landau levels in the magnetic barrier. This dip can be modulated by changing the strength and length of the magnetic barrier. By calculating the asymmetric conductance for the $K$ and ${K}^{\ensuremath{'}}$ valleys as a function of incident energy with various values of the physical parameters, optimized conditions for spin and valley polarization can be found. The results presented here provide useful information to enable implementation of spin- and valley-polarized currents in future devices.

Destructive interference and Fano antiresonance in T-shaped double quantum dots coupled to Luttinger liquid leads

We investigate the interplay of quantum interference effects and the electronic correlation on the transport through T-shaped spinless double quantum dots coupled to Luttinger liquid leads by using the nonequilibrium Green's function method. We find that the weak intralead Coulomb interaction leads to a zero-bias antiresonance dip in the differential conductance for small interdot tunneling coupling, which is distinct destructive interference characteristic. In particular, an intriguing Fano antiresonance profile for asymmetrically coupling to the Luttinger liquid leads is exhibited, consistent with the experiment findings but different physics screening origin. The strong intralead Coulomb interaction or large interdot coupling tunneling quenches the quantum interference effects, resulting in the disappearance of Fano antiresonance. The scaling behavior of differential conductance on temperature is presented for different intralead interactions. The relationship between destructive interference and two-channel Kondo effect is given. Our results provide a way of adjusting in and out of antiresonance to achieve quantum interference transistors and of switching by tuning the gate voltage controlling the interdot tunneling. • The interplay of interference and intralead Coulomb interaction on transport of T-shaped quantum dots is studied. • Weak intralead interaction leads to zero-bias dip in differential conductance for small interdot tunneling. • New physical mechanism is presented to explain for the antiresonance dip and Fano effect. • Scaling behavior of differential conductance on temperature is presented for different intralead interactions.

Evidence for unconventional superconductivity in twisted trilayer graphene.

Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity1,2. However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands3,4. Most notably in a density window spanning two to three holes per moiré unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen-Cooper-Schrieffer superconductor to a Bose-Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers5.

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.

Empty Valence‐Band Pocket in p‐Type Cu2O(111) Probed with Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopy is used to probe the surface electronic structure of (√3×√3)‐reconstructed cuprous oxide films on Au(111). In analogy to bulk Cu2O(111), the films show a pronounced p‐type nature with the valence‐band top pinned to the Fermi level and the conduction‐band onset located at +2.0 V. A conductance dip appears directly at zero bias in the dI/dV spectra, followed by an asymmetric dI/dV maximum inside the lower half of the bandgap. Several scenarios are considered to explain this unusual conductance behavior. The most likely interpretation is based on the accumulation of hole states in the p‐type material in response to the tip‐electric field and the development of an empty valence‐band pocket directly below the tip. Tunneling into these states leads to a finite dI/dV intensity even inside the Cu2O bandgap. A 1D tunneling model that accounts for this field‐dependent transport mechanism successfully reproduces the experimental data and rationalizes the impact of current setpoint, oxide thickness, and surface reconstruction on the observed spectral response.

Tunable lateral spin polarization and spin-dependent collimation in velocity-modulated ferromagnetic-gate graphene structures

The influence of spatial variation of Fermi velocity on the spin-polarized transport properties of massless Dirac fermions in proximity-induced ferromagnetic graphene junction was investigated. We found that the velocity ratio ξ causing the width of the spin conductance dip near the Dirac point is broadened as the Fermi velocity ratio increases. In contrast, the effect of the Fermi velocity mismatch does not affect the shifting of the Dirac point. It leads to the indirect measurement of the Fermi velocity ratio that can be tuned by the spin-dependent conductance. In addition to the presence of both the exchange field and the Fermi velocity modulation, we found high spin filtering occurs when the Fermi velocity ratio ξ ≥ 1. Due to the transmission probability of electron with spin down, T↓ is blocked by the effect of Fermi velocity modulation similar to waves travelling through a different media. Moreover, we also found that the resonant transmission of the electron with spin up can be perfectly transmitted through the Fermi velocity barrier at the limit injected angle θ → π/2 for H/EF ≈ 1. The spin transport properties can be controlled by suitably modifying the strength of the ferromagnetic insulator and varying the Fermi velocity modulation, which leads to the highest spin polarization. By manipulating the behavior of spin-dependent collimation and spin beam splitting in this structure, we proposed a spin transport device for making lateral spin polarization. These interesting features will help make the development of spintronic devices in the graphene-based nanostructure.

Non-Majorana states yield nearly quantized conductance in proximatized nanowires

Conductance at zero source-drain voltage bias in InSb nanowire/NbTiN superconductor devices exhibits peaks that are close to a quantized value of $2e^2/h$. The nearly quantized resonances evolve in the tunnel barrier strength, magnetic field and magnetic field orientation in a way consistent with Majorana zero modes. Our devices feature two tunnel probes on both ends of the nanowire separated by a 400 nm nanowire segment covered by the superconductor. We only find nearly quantized zero bias peaks localized to one end of the nanowire, while conductance dips are observed for the same parameters on the other end. This undermines the Majorana explanation as Majorana modes must come in pairs. We do identify states delocalized from end to end near zero magnetic field and at higher electron density, which is not in the basic Majorana regime. We lay out procedures for assessing the nonlocality of subgap wavefunctions and provide a classification of nanowire bound states based on their localization.

Modulation induced transport signatures in correlated electron waveguides

Recent transport experiments in spatially modulated quasi-1D structures created on top of LaAlO_33/SrTiO_33 interfaces have revealed some interesting features, including phenomena conspicuously absent without the modulation. In this work, we focus on two of these remarkable features and provide theoretical analysis allowing their interpretation. The first one is the appearance of two-terminal conductance plateaus at rational fractions of e^2/he2/h. We explain how this phenomenon, previously believed to be possible only in systems with strong repulsive interactions, can be stabilized in a system with attraction in the presence of the modulation. Using our theoretical framework we find the plateau amplitude and shape, and characterize the correlated phase which develops in the system due to the partial gap, namely a Luttinger liquid of electronic trions. The second observation is a sharp conductance dip below a conductance of 1\times e^2/h1×e2/h, which changes its value over a wide range when tuning the system. We theorize that it is due to resonant backscattering caused by a periodic spin-orbit field. The behavior of this dip can be reliably accounted for by considering the finite length of the electronic waveguides, as well as the interactions therein. The phenomena discussed in this work exemplify the intricate interplay of strong interactions and spatial modulations, and reveal the potential for novel strongly correlated phases of matter in systems which prominently feature both.

Quantum paracrystalline shear modes of the electron liquid

Unlike classical fluids, a quantum Fermi liquid can support a long-lived and propagating shear sound wave at arbitrarily small wave vectors and frequencies, reminiscent of the transverse sound in crystals, despite lacking any form of long-range crystalline order. This mode is expected to be present in moderately interacting metals where the quasiparticle mass is renormalized to be more than twice the bare mass in two dimensions (2D), but it has remained undetected because it is hard to excite since it does not involve charge density fluctuations, in contrast to the conventional plasma mode. In this work we propose a strategy to excite and detect this unconventional mode in clean metallic channels. We show that the shear sound is responsible for the appearance of sharp dips in the ac conductance of narrow channels at resonant frequencies matching its dispersion. The liquid resonates while minimizing its dissipation in an analogous fashion to a sliding crystal. Ultra-clean 2D materials that can be tuned towards the Wigner crystallization transition such as silicon metal-oxide-semiconductor field-effect transistors, MgZnO/ZnO, p-GaAs, and AlAs quantum wells are promising platforms to experimentally discover the shear sound.

Electron glass effects in amorphous NbSi films

We report on non equilibrium field effect in insulating amorphous NbSi thin films having different Nb contents and thicknesses. The hallmark of an electron glass, namely the logarithmic growth of a memory dip in conductance versus gate voltage curves, is observed in all the films after a cooling from room temperature to 4.2~K. A very rich phenomenology is demonstrated. While the memory dip width is found to strongly vary with the film parameters, as was also observed in amorphous indium oxide films, screening lengths and temperature dependence of the dynamics are closer to what is observed in granular Al films. Our results demonstrate that the differentiation between continuous and discontinuous systems is not relevant to understand the discrepancies reported between various systems in the electron glass features. We suggest instead that they are not of fundamental nature and stem from differences in the protocols used and in the electrical inhomogeneity length scales within each material.

Charge transport in silicene-based ferromagnetic p-wave superconductor junctions

Using Blonder-Tinkham-Klapwijk (BTK) formalism we studied the transport properties in silicene-based normal/ferromagnetic p-wave superconductor (N/FpS) junctions. Our study shows that the amplitude of the normalized conductance strongly depends on the pairing symmetry in superconductor such as px, py and px+ipy-wave symmetries, the applied electric field (EZ) and the exchange field (h) in FpS. In N/FpxS-silicene junctions the zero-bias conductance peak is obtained due to the Andreev bound states. When the electric field and magnetic field are parallel, called P configuration, there is one conductance dip (CD) for the voltage eV=lEz−λSO with λSO the spin obit coupling term and 2l the distance between two sublattices in the buckled silicene. In the anti-parallel (AP) configuration there is one CD at eV=lEz−λSO for 0<lEz−λSO<h and for lEz−λSO≥h there are two CDs at eV=h and eV=lEz−λSO, respectively. A sudden transition of conductance from zero to maximum (henceforth referred to as“switching of conductance”) at eV=lEz−λSO occurs in both N/FpyS-silicene junction and N/Fpx+ipyS-silicene junction. In N/FpyS-silicene junction the Andreev resonant peak appears at eV=h for lEz−λSO−h=0.1Δ00 in the AP case, where Δ00 is the zero temperature gap of the FpS. In N/Fpx+ipyS-silicene junctions the maximum value of the conductance appears at eV=Δ00−h in P junction while in AP junction at eV=Δ00+h.