Conductance Of Point Contact Research Articles

Electron–electron interactions in clean quantum point contacts in the large conductance regime

We report an unusual increase with temperature of the conductance of clean quantum point contact at large conductance and B = 0 T . Numerical simulation including electron–electrons interactions qualitatively explains this observation. Besides, a positive magneto-resistance that could also be related to electron–electron interactions is measured.

Universal Periods in Quantum Hall Droplets

Using the hierarchy picture of the fractional quantum Hall effect, we study the ground-state periodicity of a finite size quantum Hall droplet in a quantum Hall fluid of a different filling factor. The droplet edge charge is periodically modulated with flux through the droplet and will lead to a periodic variation in the conductance of a nearby point contact, such as occurs in some quantum Hall interferometers. Our model is consistent with experiment and predicts that superperiods can be observed in geometries where no interfering trajectories occur. The model may also provide an experimentally feasible method of detecting elusive neutral modes and otherwise obtaining information about the microscopic edge structure in fractional quantum Hall states.

The Influence of Device Geometry on Many-Body Effects in Quantum Point Contacts: Signatures of the 0.7 Anomaly, Exchange and Kondo

The conductance of a quantum point contact (QPC) shows several features that result from many-body electron interactions. The spin degeneracy in zero magnetic field appears to be spontaneously lifted due to the so-called 0.7 anomaly. Further, the g-factor for electrons in the QPC is enhanced, and a zero-bias peak in the conductance points to similarities with transport through a Kondo impurity. We report here how these many-body effects depend on QPC geometry. We find a clear relation between the enhanced g-factor and the subband spacing in our QPCs, and can relate this to the device geometry with electrostatic modeling of the QPC potential. We also measured the zero-field energy splitting related to the 0.7 anomaly, and studied how it evolves into a splitting that is the sum of the Zeeman effect, and a field-independent exchange contribution when applying a magnetic field. While this exchange contribution shows sample-to-sample fluctuations and no clear dependence on QPC geometry, it is for all QPCs correlated with the zero-field splitting of the 0.7 anomaly. This provides evidence that the splitting of the 0.7 anomaly is dominated by this field-independent exchange splitting. Signatures of the Kondo effect also show no regular dependence on QPC geometry, but are possibly correlated with splitting of the 0.7 anomaly.

Conductance of a quantum point contact based on spin-density-functional theory

We present full quantum mechanical conductance calculations of a quantum point contact (QPC) performed in the framework of the density functional theory (DFT) in the local spin-density approximation (LDA). We start from a lithographical layout of the device, and the whole structure, including semi-infinitive leads, is treated on the same footing (i.e., the electron-electron interaction is accounted for in both the leads and the central device region). We show that the spin degeneracy of the conductance channels is lifted and the total conductance exhibits a broad plateaulike feature at $\ensuremath{\sim}0.5\ifmmode\times\else\texttimes\fi{}2{e}^{2}∕h$. The lifting of the spin degeneracy is a generic feature of all studied QPC structures (both very short and very long ones, with lengths in the range $40\ensuremath{\lesssim}l\ensuremath{\lesssim}500\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$). The calculated conductance also shows a hysteresis for forward and backward sweeps of the gate voltage. These features in the conductance can be traced to the formation of weakly coupled quasibound states (magnetic impurities) inside the QPC (also predicted in previous DFT-based studies). A comparison of the results obtained with the experimental data shows, however, that while the spin-DFT-based ``first-principles'' calculations exhibit spin polarization in the QPC, the calculated conductance clearly does not reproduce the 0.7 anomaly observed in almost all QPCs of various geometries. We critically examine the major features of the standard DFT-based approach to the conductance calculations and argue that its inability to reproduce the 0.7 anomaly might be related to the infamous derivative discontinuity problem of the DFT, leading to spurious self-interaction errors not corrected in the standard LDA. Our results indicate that the formation of magnetic impurities in the QPC might be an artifact of the LDA when localization of charge is expected to occur. We thus argue that an accurate description of the QPC structure would require approaches that go beyond the standard $\mathrm{DFT}+\mathrm{LDA}$ schemes.

Hartree–Fock studies of quantum dots and the 0.7 anomaly

We apply unrestricted Hartree–Fock to modelling two systems: (1) We calculate the spin structure and addition spectra of small symmetric quantum dots (often called 2D “artificial atoms”), improving the accuracy considerably by including, for the first time, second-order correlation corrections. We compare the results to experiment and to previous numerical works, and find that our spin structure in some cases disagrees with that calculated within mean-field theories, such as Hartree–Fock without correlation corrections, or density-functional theory [C. Sloggett, O.P. Sushkov, Phys. Rev. B 71 (2005) 235326]. (2) We model the well-known 0.7 anomaly in the conductance of a quantum point contact. We calculate the conductance using direct calculation of scattering phases on a ring, within Hartree–Fock. We observe strong localisation of the Fermi electrons on the barrier, and suggest a mechanism for the observed temperature-dependent conductance anomaly.

Quantum point contact conductance in normal-metal/insulator/metal/p-wave superconductor junction

In the framework of the Blonder–Tinkham–Klapwijk model we have studied the quasiparticle transport in normal-metal/insulator/metal/p wave superconductor Junction (N/I/N/p), where the N/I/N region is a quantum wire. We calculate the quantum point contact conductance in N/I/N/p as a function of the bias voltage at zero temperature based on the Bogoliubov–de Gennes equations. Unlike the case in N/I/N/d superconductor junctions, the zero-bias conductance peaks appear in the single-mode case. The number of resonances for the quantum point contact differential conductance for the p x wave superconductor is different from that for the p y wave superconductor.

On the mechanism of hysteresis in conductance of point contacts to single ferromagnetic films

Single nonmagnetic/ferromagnetic interfaces can exhibit magnetic excitations and hysteretic switching, provided that the current density traversing the interface is sufficiently high (≳108A∕cm2) and the flow regime is diffusive. We measure hysteretic switching in conductance induced by nominally unpolarized electron currents in nanocontacts to thin Co films and successfully model the effect for ∼20nm scale point contacts using micromagnetic simulations, which take into account an out of plane stress-induced magnetic anisotropy in the point contact region.

Magneto-quantum oscillations of the conductance of a tunnel point contact in the presence of a single defect

The influence of a strong magnetic field $H$ to the conductance of a tunnel point contact in the presence of a single defect has been considered. We demonstrate that the conductance exhibits specific magneto-quantum oscillations, the amplitude and period of which depend on the distance between the contact and the defect. We show that a nonmonotonic dependence of the point-contact conductance results from a superposition of two types of oscillations: A short period oscillation arising from the electrons being focused by the field $H$ and a long period oscillation originated from the magnetic flux passing through the closed trajectories of electrons moving from the contact to the defect and returning back to the contact.

Pulsed measurements of the nonlinear conductance of quantum point contacts

The conductance of quantum point contacts (QPCs) subject to strongly nonlinear source-drain biasing is investigated with transient pulses. The authors investigations reveal the presence of a characteristic fixed point, at which the transient conductance (Gt) is bias independent. This point corresponds to the situation where the unbiased QPC is almost depopulated and can apparently be accounted for by considering the unidirectional population of QPC subbands by the transient voltage. To discuss the variations of Gt away from the fixed point, it is necessary to consider the influence of the applied bias on the QPC profile and electron-phonon scattering.

Quantized conductance without reservoirs: Method of the nonequilibrium statistical operator

We introduce a generalized non-equilibrium statistical operator (NSO) to study a current-carrying system. The NSO is used to derive a set of quantum kinetic equations based on quantum mechanical balance equations. The quantum kinetic equations are solved self-consistently together with Poisson’s equation to solve a general transport problem. We show that these kinetic equations can be used to rederive the Landauer formula for the conductance of a quantum point contact, without any reference to reservoirs at different chemical potentials. Instead, energy dissipation is taken into account explicitly through the electron-phonon interaction. We find that both elastic and inelastic scattering are necessary to obtain the Landauer conductance.

From quantum point contacts to quantum wires: Density-functional calculations with exchange and correlation effects

We numerically analyze the conductance and spin polarization of realistic quantum point contacts (QPCs) using density-functional theory, including both exchange and correlation effects. The self-consistent calculations are performed as a function of split gate voltage, for different temperatures and QPC lengths.We show that in short enough QPCs $(100\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ there is no spontaneous spin polarization, and the conductance for up-spin and down-spin electrons is the same. As the length of the QPC increases, so does the spin polarization and the difference in conductance between up-spin and down-spin electrons, resulting in an anomalous structure in the total conductance---the 0.7 anomaly. This structure moves from around 0.9 (in units of $2{e}^{2}∕h$) for a $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ QPC to slightly below 0.5 for a $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ QPC. Due to the strong ferromagnetic spin polarization in a long QPC, it will effectively work as a spin filter. The temperature dependence of the conductance is discussed in relation to the ``Reilly model,'' whose underlying assumption, regarding the shape of the spin gaps, is investigated using the self-consistent results.

Resonantly Enhanced Nonlinear Conductance in Long Quantum Point Contacts near Pinch-Off

We report on a remarkable resonance in the differential conductance of long quantum point contacts (QPCs) that is observed as a precursor to regular quantized transport. This effect is increasingly pronounced in longer QPCs, in which the differential conductance may resonantly exceed 2e2/h. From a study of the experimental characteristics of this feature, we suggest that it may be associated with the formation of a well-resolved energy gap that opens dynamically as a result of enhanced many-body interactions in long QPCs.

Quantum point contact with a large localized spin: Fractional quantization of the ballistic conductance

We analyze the conductance of the quantum point contact containing large localized spin J. The additional plateau is formed on a ballistic conductance staircase if only one propagating channel is rendered conducting. The conductance value at this plateau is shown to depend strongly on J and decrease from 3e^2/2h to e^2/h when J increases from 1/2 to infinity, which is in a good agreement with the experimental observations [D.J. Reilly, et al, Phys. Rev. B 63, 121311 (2001)].

Signature of Fermi-surface anisotropy in point contact conductance in the presence of defects

In a previous paper (Avotina et al.,Phys. Rev. B Vol.71, 115430 (2005)) we have shown that in principle it is possible to image the defect positions below a metal surface by means of a scanning tunnelling microscope. The principle relies on the interference of electron waves scattered on the defects, which give rise to small but measurable conductance fluctuations. Whereas in that work the band structure was assumed to be free-electron like, here we investigate the effects of Fermi surface anisotropy. We demonstrate that the amplitude and period of the conductance oscillations are determined by the local geometry of the Fermi surface. The signal results from those points for which the electron velocity is directed along the vector connecting the point contact to the defect. For a general Fermi surface geometry the position of the maximum amplitude of the conductance oscillations is not found for the tip directly above the defect. We have determined optimal conditions for determination of defect positions in metals with closed and open Fermi surfaces.

Hysteretic effects in lateral nanostructures caused by long-lived quantum-Hall eddy currents

Coulomb blockade studies of lateral quantum dots and measurements of the quantised conductance of quantum point contacts, in high magnetic fields, reveal novel features which are hysteretic in magnetic-field sweep direction. These features are associated with long-lived eddy currents, induced in the 2D electron gas leads in these devices as the magnetic field sweeps and the 2DEG enters the quantum Hall effect state. Torsion-balance magnetometry measurements confirm the presence of these induced currents, and their influence on the nanostructures. The decay of the eddy currents, after the magnetic field sweep is stopped, exhibits two distinct regimes: a fast initial exponential decay followed by a much longer power-law decay in which the size of the eddy current falls typically to 60% of its original value in one day. The interpretation of these observations is that the Coulomb blockade and quantum point contact devices are influenced by the local Hall potential at the edges of the 2DEG leads. The Hall potential changes the local chemical potential of the leads, and hence the properties of the devices, in a manner which reverses when the field sweep direction is reversed.

Quantized Conductance in Atomic-Scale Point Contacts Formed by Local Electrochemical Deposition of Silver

not Available.

Linear conductance of quantum point contacts with deliberately broken symmetry

We investigate the linear transport properties of quantum point contacts (QPCs) whosesymmetry is deliberately broken in a controlled manner. The devices that we study consistof a conventional split-gate QPC, which is modified by the inclusion of an additionalperturbing gate that is used to modulate the electron density on one side of the device. Asthe voltage applied to this ‘finger gate’ is varied, we observe several reproduciblefeatures below the last integer plateau, as well as strong modifications of theinteger-plateau staircase. Self-consistent calculations, performed for the exact devicestructure utilized in experiment, suggest that these features are related to the abilityof the finger gate to strongly disrupt the symmetry of the QPC, reducing theelectron density significantly on one side of the device. We discuss these resultswithin the context of recent models for many-body electron transport in QPCs.

Low-temperature phonon transport in 3D point-contacts (Review)

This review is devoted to describing nonequilibrium carrier systems and relaxational and kinetic phenomena in three-dimensional point-contacts. Attention is focused on describing a phonon system which becomes substantially modified under conditions of ballistic transport. In such systems the energy fluxes are limited by the presence of weakly coupled layers of impurity atoms, planar defects, or microscopic-size contacts. The small size of point-contacts, ranging from several to 1000 nm, makes it possible to investigate low-temperature heat and charge transfer on scales less than the characteristic inelastic scattering lengths. A mechanism of phonon transport in the presence of an interface is analyzed, and various models of a planar defect are examined. The special features of interfacial phonon transport, where the transport coefficients are determined not by scattering processes in the volume of a bulk crystal but rather by the properties of the intercrystalline boundary, are studied. The quantum phonon thermal conductivity of point-contacts is studied in detail.

Correlated quantization of supercurrent and conductance in a superconducting quantum point contact

We have measured the supercurrent and conductance of a superconducting quantum point contact in a superconductor two-dimensional electron gas-superconductor Josephson junction. We observe that the supercurrent and conductance change stepwise in a correlated manner as a function of the gate voltage. This was achieved by simultaneous measurement of the supercurrent and conductance at high bias from the same current voltage characteristic.

Method to determine defect positions below a metal surface by STM

The oscillatory voltage dependence of the conductance of a quantum point contact in the presence of a single point-like defect has been analyzed theoretically. Such signals are detectable and may be exploited to obtain information on defect positions below a metal surface. Both tunnel junctions and ballistic contacts of adiabatic shape have been considered. The effect of quantum interference has been taking into account between the principal wave that is directly transmitted through the contact and the partial wave that is scattered by the contact and the defect. This effect leads to oscillations of the conductance as a function of applied voltage. We obtain the dependence of the period and amplitude of the conductance oscillations on the position of the defect inside the metal.