Conductance Of Point Contact Research Articles

Portable Device for Multipurpose Research on Dendritic Yanson Point Contacts and Quantum Sensing.

Quantum structures are ideal objects by which to discover and study new sensor mechanisms and implement advanced approaches in sensor analysis to develop innovative sensor devices. Among them, one of the most interesting representatives is the Yanson point contact. It allows the implementation of a simple technological chain to activate the quantum mechanisms of selective detection in gaseous and liquid media. In this work, a portable device for multipurpose research on dendritic Yanson point contacts and quantum sensing was developed and manufactured. The device allows one to create dendritic Yanson point contacts and study their quantum properties, which are clearly manifested in the process of the electrochemical cyclic switchover effect. The device tests demonstrated that it was possible to gather data on the compositions and characteristics of the synthesized substances, and on the electrochemical processes that influence the production of dendritic Yanson point contacts, as well as on the electrophysical processes that provide information on the quantum nature of the electrical conductance of dendritic Yanson point contacts. The small size of the device makes it simple to integrate into a micro-Raman spectrometer setup. The developed device may be used as a prototype for designing a quantum sensor that will serve as the foundation for cutting-edge sensor technologies, as well as be applied to research into atomic-scale junctions, single-atom transistors, and any relative subjects.

Multiwell Potential in a Trench-Type Quantum Point Contact

A physical mechanism has been proposed to explain the appearance of the multichannel electron transport regime in trench quantum point contacts. It has been shown that the redistribution of electrons in a two-dimensional electron gas, which is due to the Coulomb interaction between them and with charged donors and X-valley electrons, results in the experimentally observed multichannel regime. The numerical simulation of the redistribution of electrons and its comparison with the measurements of the conductance of trench quantum point contacts based on a GaAs/AlGaAs heterostructure demonstrate the formation of a multiwell confining potential.

Large Zeeman Splitting in Out-of-Plane Magnetic Field in a Double-Layer Quantum Point Contact

In this study, we observe that the conductance of a quantum point contact on a GaAs/AlGaAs double quantum well depends significantly on the magnetic field perpendicular to the two-dimensional electron gas. In the presence of the magnetic field, the subband edge splitting due to the Zeeman energy reaches 0.09 meV at 0.16 T, thereby suggesting an enhanced g-factor. The estimated g-factor enhancement is 17.5 times that of the bare value. It is considered that a low electron density and high mobility makes it possible to reach a strong many-body interaction regime in which this type of strong enhancement in g-factor can be observed.

Nodeless superconductivity in the charge density wave superconductor LaPt2Si2

We have studied the superconducting gap structure of LaPt$_2$Si$_2$ by measuring the temperature dependence of the London penetration depth shift $\Delta\lambda(T)$ and point contact spectroscopy of single crystals. $\Delta\lambda(T)$ shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density $\rho_{s}(T)$ can be well described by a single-gap s-wave model. The point-contact conductance spectra can also be well fitted by an s-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II superconductivity. These results suggest fully gapped superconductivity in LaPt$_2$Si$_2$, with moderately strong electron-phonon coupling.

Crossing and anticrossing of 1D subbands in a quantum point contact with in-plane side gates

The conductance of a single GaAs quantum point contact (QPC) with in-plane side gates separated from a channel by lithographic trenches is experimentally studied. It is shown that, in such a trench-type QPC, the conducting channel splits into two channels, each demonstrating a conductance quantization. A phase diagram of the 1D subband population in separate channels as a function of the voltages at side gates is experimentally obtained. The obtained data show that the electron transport regime undergoes the transition from the case of two independent parallel 1D conductors to the case of strongly correlated double-row transport. The transition occurs with the increase in the number of populated 1D subbands in the channels and manifests itself in the phase diagram by the change of behavior of the lines separating the regions corresponding to different numbers. The crossings of these lines at small numbers turn into anticrossings at large numbers. The anticrossings are explained by the splitting of degenerate levels in a double-well confinement potential formed inside the QPC constriction.

Double-Channel Electron Transport in Suspended Quantum Point Contacts with in-Plane Side Gates

The conductance of a suspended quantum point contact fabricated on the basis of GaAs/AlGaAs heterostructures with a two-dimensional electron gas and equipped with side gates separated from the constriction using lithographical trenches is investigated. The gate dependences of the conductance of such structures correspond to the unusual double-channel mode with independent quantization of the conductance of two channels, and the conductance of separate channels can be independently controlled using two side gates. The electrostatic-formation mechanism of a two-channel structure inside a single constriction associated with the lateral redistribution of low-mobility X-valley electrons containing superlattice heterostructure layers, which leads to the formation of a potential barrier separating the conductivity electrons into two channels symmetrically shifted towards the lithographic trenches, specifying the nanostructure geometry, is considered.

Effective Lorenz Number of the Point Contact between Silver Nanowires.

Electrical and thermal transport through metal point contacts, a key issue in the design and operation of various engineering devices, is of great recent interest. The effective Lorenz number (L), which relates the thermal to electrical conductance of point contacts, could provide valuable information on the relative contribution of electrons and phonons to thermal transport. Through measuring electrical and thermal transport across point contacts between silver nanowires, we report that L significantly deviates from the Sommerfeld value by up to 5.2 times and exhibits nonmonotonic variation with temperature. Analyses show that these observations are due to the more important phonon contribution to the thermal conductance of the point contact as Sharvin resistance greatly hinders electron transport, which is further confirmed by the size dependence of L with a higher value for a smaller contact size. These results provide critical insights into engineering designs involving point contacts between metal nanostructures.

Selective detection of complex gas mixtures using point contacts: concept, method and tools.

Of all modern nanosensors using the principle of measuring variations in electric conductance, point-contact sensors stand out in having a number of original sensor properties not manifested by their analogues. The nontrivial nature of point-contact sensors is based on the unique properties of Yanson point contacts used as the sensing elements. The quantum properties of Yanson point contacts enable the solution of some of the problems that could not be solved using conventional sensors measuring conductance. In the present paper, we demonstrate this by showing the potential of quantum point-contact sensors to selectively detect components of a gas mixture in real time. To demonstrate the high efficiency of the proposed approach, we analyze the human breath, which is the most complex of the currently known natural gas mixtures with extremely low concentrations of its components. Point-contact sensors allow us to obtain a spectroscopic profile of the mixture. This profile contains information about the complete set of energy interactions occurring in the point contact/breath system when the breath constituents adsorb to and desorb from the surface of the point-contact conduction channel. With this information we can unambiguously characterize the analyzed system, since knowing the energy parameters is key to successfully identifying and modeling the physicochemical properties of various quantum objects. Using the point-contact spectroscopic profile of a complex gas mixture it is possible to get a functional dependence of the concentration of particular breath components on the amplitude of the sensor output signal. To demonstrate the feasibility of the proposed approach, we analyze the point-contact profiles from the breath of several patients and compare them with the concentrations of serotonin and cortisol in the body of each patient. The obtained results demonstrate that the proposed methodology allows one to get an effective calibration function for a non-invasive analysis of the level of serotonin and cortisol in the human body using the point-contact breath test. The present study indicates some necessary prerequisites for the design of fast detection methods using differential sensor analysis in real time, which can be implemented in various areas of science and technology, among which medicine is one of the most important.

Observation of the Spin-Orbit Gap in Bilayer Graphene by One-Dimensional Ballistic Transport.

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 μeV 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 it in the other one. In addition, we observe a spin polarized channel of 6e^{2}/h at high in-plane magnetic field and signatures of interaction effects at the crossings of spin-split subbands of opposite spins at finite magnetic field.

Двухканальный электронный транспорт в подвешенных квантовых точечных контактах с боковыми затворами

The conductance of a suspended quantum point contact fabricated from GaAs/AlGaAs heterostructures with a two-dimensional electron gas, equipped with the in-plane side gates separated from the constriction using lithographical trenches, is studied. The conductance as a function of the gate voltages demonstrates unusual double-channel regime with independent channel’s conductance quantization: two side gates can drive the conductance of the separate channels independently. A possible electrostatic mechanism of the double-channel structure formation inside a single constriction is connected with the lateral redistribution of the low-mobility X-valley electrons contained in superlattice layers, resulting in the emergence of the potential barrier in the middle of quantum point contact, separating the conducting electrons into two channels, symmetrically shifted towards the lithographical trenches, defining the nanostructure geometry.

Quantum sensor of new generation

The aim of this paper was to develop a prototype of new quantum sensor whose sensing element is a Yanson point contact. The exceptional features of quantum point-contact sensors, besides the proper quantum properties of Yanson point contacts, are due to a number of their fundamental properties. These properties include the specific potential distribution in the contact contributing to the appearance of the point-contact gas-sensitive effect and the gapless electrode system formed in the point-contact conduction channel in a liquid medium and responsible for the cyclic switchover effect which makes it possible to record a broad spectrum of quantum states in dendritic Yanson point contacts placed in the analyzed media. Conductance histograms corresponding to the quantum states of dendritic point contacts undergoing transformations are markers of certain media and could be recorded with the prototype of new quantum sensor developed in our paper. The efficiency of the developed prototype was demonstrated in experiments with gaseous media of argon and ambient air. The obtained results lay the foundations for the development of new generation of quantum sensors for selective detection in liquid and gaseous media.

Resistive Switching and Charge Transport in Laser-Fabricated Graphene Oxide Memristors: A Time Series and Quantum Point Contact Modeling Approach.

This work investigates the sources of resistive switching (RS) in recently reported laser-fabricated graphene oxide memristors by means of two numerical analysis tools linked to the Time Series Statistical Analysis and the use of the Quantum Point Contact Conduction model. The application of both numerical procedures points to the existence of a filament connecting the electrodes that may be interrupted at a precise point within the conductive path, resulting in resistive switching phenomena. These results support the existing model attributing the memristance of laser-fabricated graphene oxide memristors to the modification of a conductive path stoichiometry inside the graphene oxide.

Non-integer spin-polarized conductance in multichannel magnetic point contacts

The ballistic conductance of a geometrically confined head-to-head 180° domain wall at a magnetic point contact in a two-dimensional (2D) metallic nanowire is studied theoretically. The ballistic conductance is computed by introducing the modified Landauer-Buttiker formalism in which the spin polarization effects are included. The analysis is based on coherent scattering of the carriers by the spin-dependent potential associated with the wall structure. The transmission properties of coherent states are obtained by introducing an algorithm to solve the coupled spin channels Schrodinger equation with mixed Dirichlet-Neumann boundary conditions applied far from the domain wall. Non-integer values for the conductance have been observed, due to the modified chemical potentials at the ferromagnetic contact probes. It has been shown that the dips in conductance arise when the electrons are partially polarized in the ferromagnetic leads, due to the formation of spin-dependent chemical potential.

Conductance quantization as a new selective sensing mechanism in dendritic point contacts

The diversity of techniques employed in modern sensing nanodevices is crucial for large-scale use of sensors in multifunctional technological cycles. We propose a new concept of selective detection of gases and liquids based on the formation of an original quantum system and registration of its energy states in dynamic mode using dendrite point contacts synthesized electrochemically in the probed medium. The in situ synthesis of nanosized dendrite point contacts is shaped by the cyclic switchover effect which takes place in an electrolyte in contact with the analyzed medium and results in consecutive cycles of the formation and destruction of an electrochemical gapless electrode system. Conductivity of such point contacts demonstrates quantum behavior driven by the shell effect which determines the geometry of their conducting channels. Temporal dependence of dendrite point contact electrical resistance measured in dynamic mode is characterized by a step-like structure which reflects the metastable quantum states of the system whose distribution can be presented in the form of a conductance histogram. The histogram is a unique fingerprint of the probed medium and can thus be used to unambiguously identify it. The dynamic mode scanning of the energy states of point contact quantum systems proposed here makes it possible to develop a universal method for selective detection of many gaseous and liquid media including such hard to detect substances as methane and rare gases. The new approach is expected to prove its efficiency in investigating quantum effects for various sensor applications and stimulate the development of the next generation of highly selective nanodevices. The new concept of selective detection of gases and liquids is based on the registration of quantum states of nanosized dendrite point contacts synthesized in the probed medium.

A single full gap in noncentrosymmetric superconductor Re6Hf by point-contact spectroscopy

We report a point-contact spectroscopy study on the noncentrosymmetric superconductor Re6Hf with a superconducting transition temperature Tc ∼ 6.0 K. The point-contact conductance spectra can be well fitted by an s-wave Blonder-Tinkham-Klapwijk (BTK) model, where the superconducting gap follows a standard BCS behavior as a function of temperature or magnetic field. The gap at 0 K is estimated to be Δ0 ∼ 0.92 meV, yielding 2Δ0/kBTc ∼ 3.56. Hence, our results are consistent with a fully gaped s-wave pairing symmetry with a negligible spin-triplet superconducting component in Re6Hf.

Steps of the Giant Terahertz Photoconductance of a Tunneling Point Contact

The photoresponse of the conductance of a tunneling quantum point contact to irradiation in the frequency range of 0.5–1.7 THz has been simulated within the theory of photon-assisted tunneling. The simulation has shown that the dependences of the conductance on the Fermi energy exhibit steps corresponding to transitions with the absorption of one, two, or three photons to channels near the top of the tunnel barrier. Under experimentally achievable conditions, the ratio of the photoconductance to the dark value on these steps can reach several orders of magnitude.

Lateral-electric-field-induced spin polarization in a suspended GaAs quantum point contact

The conductance of a GaAs-based suspended quantum point contact (QPC) equipped with lateral side gates has been experimentally studied in the absence of the external magnetic field. The half-integer conductance plateau (0.5×2e2/h) has been observed when an asymmetric voltage between the side gates is applied. The appearance of this plateau has been attributed to the spin degeneracy lifting caused by the spin-orbit coupling associated with the lateral electric field in the asymmetrically biased QPC. We have experimentally demonstrated that, despite the relatively small g-factor in GaAs, the observation of the spin polarization in the GaAs-based QPC became possible after the suspension due to the enhancement of the electron-electron interaction and the effect of the electric field guiding. These features are caused by a partial confinement of the electric field lines within a suspended semiconductor layer with a high dielectric constant.

Electrical control of charged carriers and excitons in atomically thin materials.

Electrical confinement and manipulation of charge carriers in semiconducting nanostructures are essential for realizing functional quantum electronic devices1-3. The unique band structure4-7 of atomically thin transition metal dichalcogenides (TMDs) offers a new route towards realizing novel 2D quantum electronic devices, such as valleytronic devices and valley-spin qubits 8 . 2D TMDs also provide a platform for novel quantum optoelectronic devices9-11 due to their large exciton binding energy12,13. However, controlled confinement and manipulation of electronic and excitonic excitations in TMD nanostructures have been technically challenging due to the prevailing disorder in the material, preventing accurate experimental control of local confinement and tunnel couplings14-16. Here we demonstrate a novel method for creating high-quality heterostructures composed of atomically thin materials that allows for efficient electrical control of excitations. Specifically, we demonstrate quantum transport in the gate-defined, quantum-confined region, observing spin-valley locked quantized conductance in quantum point contacts. We also realize gate-controlled Coulomb blockade associated with confinement of electrons and demonstrate electrical control over charged excitons with tunable local confinement potentials and tunnel couplings. Our work provides a basis for novel quantum opto-electronic devices based on manipulation of charged carriers and excitons.

Electric-field control of conductance in metal quantum point contacts by electric-double-layer gating

An electric-field control of quantized conductance in metal (gold) quantum point contacts (QPCs) is demonstrated by adopting a liquid-gated electric-double-layer (EDL) transistor geometry. Atomic-scale gold QPCs were fabricated by applying the feedback-controlled electrical break junction method to the gold nanojunction. The electric conductance in gold QPCs shows quantized conductance plateaus and step-wise increase/decrease by the conductance quantum, G0 = 2e2/h, as EDL-gate voltage is swept, demonstrating a modulation of the conductance of gold QPCs by EDL gating. The electric-field control of conductance in metal QPCs may open a way for their application to local charge sensing at room temperature.

Scanning Gate Microscope for Cold Atomic Gases.

We present a scanning probe microscopy technique for spatially resolving transport in cold atomic gases, in close analogy with scanning gate microscopy in semiconductor physics. The conductance of a quantum point contact connected to two atomic reservoirs is measured in the presence of a tightly focused laser beam acting as a local perturbation that can be precisely positioned in space. By scanning its position and recording the subsequent variations of conductance, we retrieve a high-resolution map of transport through a quantum point contact. We demonstrate a spatial resolution comparable to the extent of the transverse wave function of the atoms inside the channel and a position sensitivity below 10nm. Our measurements agree well with an analytical model and abinitio numerical simulations, allowing us to identify a regime in transport where tunneling dominates over thermal effects. Our technique opens new perspectives for the high-resolution observation and manipulation of cold atomic gases.