Mesoscopic Junctions Research Articles

Energy conductance across a mesoscopic junction in a nonequilibrium spin-boson model under the influence of telegraph noise.

A spin-boson model in the presence of a telegraph noise (TN) source is employed to calculate the energy conductance between a tunnel junction and two bosonic baths. A polaron-transformed coupling term with the bosonic baths allows for treating quantum damping to arbitrary orders of strength. However, the polaron transformation yields a dressed tunneling frequency which is assumed small and treated perturbatively as is familiar in the noninteracting blip approximation in the context of the nonequilibrium spin-boson model. While the coupling with the bosonic baths leads to decoherence in an otherwise coherent tunneling process, the TN induces a different kind of fluctuation, that is, in the asymmetry of the underlying two-level system. It is the interplay of these two different relaxation effects, one triggered by the two quantum (bosonic) baths and the other through a classical bath (creating a TN), that is investigated here in detail. The TN that mimics the classical, fluctuating environment makes a nontrivial contribution to the self-energy that helps compute the imaginary part of the spin susceptibility which, in turn, determines the energy transfer across the junction. The range of validity of the TN is clarified at the outset and its efficacy in tuning the environmental influence is pointed out. The present paper complements an earlier similar study-albeit for fermionic baths-and provides additional input in terms of the TN to a previous investigation of energy transfer between a nanojunction and bosonic reservoirs without, however, the noisy environment.

Graphite superlubricity enabled by triboinduced nanocontacts

Colloidal probe Atomic Force Microscopy allows to explore sliding states of vanishing friction, i.e. superlubricity, in mesoscopic graphite contacts. Superlubricity is known to appear upon formation of a triboinduced transfer layer, originated by material transfer of graphene flakes from the graphitic substrate to the colloidal probe. Previous studies suggest that friction vanishes due to crystalline incommensurability at the newly formed interface. However this picture still lacks several details, such as the roles of the tribolayer roughness and of loading conditions. Hereafter we gain deeper insight into the tribological response of micrometric silica beads sliding on graphite under ambient conditions. We show that the tribotransferred flakes behave as lubricious nanoasperities with a twofold role. First, they decrease the silica-graphite true contact area, in fact causing a breakdown of adhesion and friction by one order of magnitude. Second, they govern mechanical dissipation through the specific energy landscape experienced by the topographically-highest triboinduced nanoasperity. Remarkably, such contact junctions can undergo a load-driven atomic-scale transition from continuous superlubric sliding to dissipative stick-slip, that agrees with the single-asperity Prandtl-Tomlinson model. Superlubricity in mesoscopic silica-graphite junctions may therefore arise from the load-controlled competition between interfacial crystalline incommensurability and contact pinning effects at one dominant nanoasperity.

Continuous-Variable Entanglement Test in Driven Quantum Contacts.

The standard entanglement test using the Clauser-Horne-Shimony-Holt inequality is known to fail in mesoscopic junctions at finite temperatures. Since this is due to the bidirectional particle flow, a similar failure is expected to occur in an ac-driven contact. We develop a continuous-variable entanglement test suitable for electrons and holes that are created by the ac drive. At low enough temperatures the generalized Bell inequality is violated in junctions with low conductance or a small number of transport channels and with ac voltages which create few electron-hole pairs per cycle. Our ac-entanglement test depends on the total number of electron-hole pairs and on the distribution of probabilities of pair creations similar to the Fano factor.

Shot noise of charge and spin transport in a junction with a precessing molecular spin

Magnetic molecules and nanomagnets can be used to influence the electronic transport in mesoscopic junction. In a magnetic field the precessional motion leads to resonances in the dc- and ac-transport properties of a nanocontact, in which the electrons are coupled to the precession. Quantities like the dc-conductance or the ac-response provide valuable information like the level structure and the coupling parameters. Here, we address the current noise properties of such contacts. This encompasses the charge current and spin-torque shot noise, which both show a step-like behavior as functions of bias voltage and magnetic field. The charge current noise shows pronounced dips around the steps, which we trace back to interference effects of electron in quasienergy levels coupled by the molecular spin precession. We show that some components of the noise of the spin-torque currents are directly related to the Gilbert damping and, hence, are experimentally accessible. Our results show that the noise characteristics allow to investigate in more detail the coherence of spin transport in contacts containing magnetic molecules.

Impurity states in mesoscopic SNS junctions with a point defect

We study theoretically localized states in a clean three-dimensional SNS junction, containing one point defect (impurity) located in the normal layer. We find that the defect induces two quasibound states in a short junction, corresponding to different spin projections. The energies of these states coincide for a nonmagnetic defect and are generally different for a magnetic defect. The spatial structures of the impurity states resemble the structures of Shiba states in a bulk superconductor. We also consider the case when the impurity is located close to a fat edge of the junction. We find mesoscopic fluctuations of the energies of the quasibound states as a function of the distance between the impurity and the edge.

Ginzburg–Landau theory of mesoscopic multi-band Josephson junctions

A Ginzburg–Landau theory for multi-band mesoscopic Josephson junctions has been developed. The theory, obtained by generalizing the de Gennes matching-matrix method for the interface order parameters, allows the study of the phase dynamics of various types of mesoscopic Josephson junctions. As a relevant application, we studied mesoscopic double-band junctions also in the presence of a superconducting nanobridge interstitial layer. The results are in agreement with a microscopic treatment of the same system. Furthermore, thermal stability of the nanobridge junction is discussed in connection with recent experiments on iron-based grain-boundary junctions.

Electron and electron–hole excitations in a driven Fermi sea

AbstractWe study the response of a degenerate Fermi sea to a time‐dependent perturbation. We find that elementary excitations in the Fermi sea are electrons and electron–hole quasiparticle pairs whose number and probability of creation depend on amplitude and shape of the drive. The excitations can be probed in a driven quantum contact at low temperature. While electronic excitations are associated to the dc voltage offset and determine the average current, the electron–hole pairs are created by the ac voltage component and give rise to the excess current noise power in a junction. We also obtain the many‐body state created by the drive. The constituent quasiparticle states can be probed by a Hong–Ou–Mandel type of correlation experiment where electrons emitted from the terminals with a relative time delay collide at the contact. The current noise power in the junction is directly related to the wave packet overlap at the contact. This is confirmed in recent experiments, demonstrating an unprecedented degree of control of elementary excitations in a coherent mesoscopic junction.

Onset of superconductivity in a voltage-biased normal-superconducting-normal microbridge

We study the stability of the normal state in a mesoscopic NSN junction biased by a constant voltage V with respect to the formation of the superconducting order. Using the linearized time-dependent Ginzburg-Landau equation, we obtain the temperature dependence of the instability line, V_{inst}(T), where nucleation of superconductivity takes place. For sufficiently low biases, a stationary symmetric superconducting state emerges below the instability line. For higher biases, the normal phase is destroyed by the formation of a non-stationary bimodal state with two superconducting nuclei localized near the opposite terminals. The low-temperature and large-voltage behavior of the instability line is highly sensitive to the details of the inelastic relaxation mechanism in the wire. Therefore, experimental studies of V_{inst}(T) in NSN junctions may be used as an effective tool to access parameters of the inelastic relaxation in the normal state.

Shot noise of multichannel transport in mesoscopic junction systems

Shot noise, which accompanies the emergence of the quantization nature (discreteness) of electrons, indicates dynamic fluctuations in electrical current and further reveals underlying operations in mesoscopic applications. This work presents a multichannel junction device with which to study the shot noise of coupled-channel transports as in normal nanocircuits, and it describes the theoretical algorithms. The numerical results show that, in contrast to the indistinguishable results of average current on direct-current measurements, the shot noise related to a dynamic current definitely exhibits significant differences among devices with diverse multichannel set-ups and presents super-Poissonian behaviors associated with the splitting of tunneling states.

Model for large arrays of Josephson junctions with unconventional superconductors

We study large arrays of mesoscopic junctions made out of gapless unconventional superconductors where the tunneling processes of both particle-hole and Cooper pairs give rise to a strongly retarded effective action which, contrary to the standard case, cannot be readily characterized in terms of a local Josephson energy. This action can be relevant, for example, to grain boundary and $c$-axis junctions in layered high-${T}_{c}$ superconductors. By using a particular functional representation, we describe emergent collective phenomena in this system, ascertain its phase diagram, and compute electrical conductivity.

Fabrication, electrical characterization, and modeling of fully-porous pn junctions

In this work, fabrication, electrical characterization, and modeling of fully porous pn junctions (FPJs) consisting of elemental mesoscopic crystalline junctions operating in parallel, is presented. FPJs are fabricated by anodic etching of a pn crystalline substrate and show a rectifying behavior strongly dependent on PS surface termination, as proved by electrical measurements performed after both room-temperature aging and thermal-oxidation treatment. Modeling of FPJs is performed using a lumped equivalent circuit consisting of a diode, taking into account the elemental mesoscopic junctions, and two resistances acting one in series and one in parallel to the diode, the latter taking into account conduction paths at the silicon mesocrystal surface. Best fitting of experimental data results in good agreement between theoretical and measured I-V curves, thus corroborating the proposed model.

Proposal for a cumulant-based Bell test for mesoscopic junctions

The creation and detection of entanglement in solid state electronics is of fundamental importance for quantum information processing. We prove that second-order quantum correlations can be always interpreted classically and propose a general test of entanglement based on the violation of a classically derived inequality for continuous variables by fourth-order quantum correlation functions. Our scheme provides a way to prove the existence of entanglement in a mesoscopic transport setup by measuring higher order cumulants without requiring the additional assumption of a single-charge detection.

Quasiprobabilistic Interpretation of Weak Measurements in Mesoscopic Junctions

The impossibility of measuring noncommuting quantum mechanical observables is one of the most fascinating consequences of the quantum mechanical postulates. Hence, to date the investigation of quantum measurement and projection is a fundamentally interesting topic. We propose to test the concept of weak measurement of noncommuting observables in mesoscopic transport experiments, using a quasiprobabilistic description. We derive an inequality for current correlators, which is satisfied by every classical probability but violated by high-frequency fourth-order cumulants in the quantum regime for experimentally feasible parameters.

Electrical Control of Magnetic States

Charge and spin transport in artificial magnetic structures, like nanopillar spin valves, planar tunnel junctions, and mesoscopic double-barrier junctions (single-electron transistors) are briefly surveyed, with particular emphasis on magnetoresistance effects and current-induced magnetic switching and spin dynamics. The methods of spin current generation are also discussed, particularly the method based on spin Hall effect.

Electron tunneling through a hybrid superconducting-normal mesoscopic junction under microwave radiation

We present a theoretical analysis for electron tunneling through a hybrid superconducting-normal mesoscopic junction consisting of a superconducting electrode, a two-level quantum dot, and a normal electrode under single mode microwave radiation. Using nonequilibrium Green's-function formalism, we incorporate Floquet basis in the Nambu space and solve the Green's function with finite matrix truncation to obtain the transport properties numerically. We studied the effects of photon-induced single-level oscillations and quantum transition between levels on the time-averaged current-voltage $(I\text{\ensuremath{-}}V)$ characteristics of the system. For quantum dot with a single localized level, the main dc resonance remains unchanged regardless of the frequency and amplitude of the radiation, and a series of secondary resonances due to multiphoton processes are present. For quantum dot with two localized levels, the sole effects from the transitions between levels produce splitting on the main dc resonance at Rabi frequency proportional to the coupling. This provides the possibility for experimental inference of the interlevel coupling strength of the driven resonant tunneling system from the bias voltage energy difference between the split resonances in the $I\text{\ensuremath{-}}V$ curve.

Models of mesoscopic time-resolved current detection

Quantum transport in mesoscopic conductors is essentially governed by the laws of quantum mechanics. One of the major open questions of quantum mechanics is what happens if non-commuting observables are measured simultaneously. Since current operators at different times do not commute, the high-frequency correlation functions of the current are realization of this fundamental quantum question. We formulate this problem in the context of measurements of finite-frequency current cumulants in a general quantum point contact, which are the subject to ongoing experimental effort. To this end, we present two models of detectors that correspond to a weak time-resolved measurement of the electronic current in a mesoscopic junction. In both cases, the backaction of the detector leads to observable corrections to the current correlations functions involving the so-called noise susceptibilities. As a result, we propose a reinterpretation of environmental corrections to the finite-frequency cumulants as inevitable effect resulting from basic quantum mechanical principles. Finally we make concrete predictions for the temperature-, voltage-, and frequency-dependence of the third cumulant, which could be verified directly using current experimental techniques.

Josephson current in nanofabricated V/Cu/V mesoscopic junctions

We report the fabrication of planar V/Cu/V mesoscopic Josephson weak links of different size and the analysis of their low-temperature behavior. The shorter junctions exhibit critical currents of several tens of microamperes at 350 mK, while Josephson coupling persists up to ∼2.7 K. Good agreement is obtained by comparing the measured switching currents to a model which holds in the diffusive regime. Our results demonstrate that V is an excellent candidate for the implementation of superconducting nanodevices operating at a few kelvins.

Magneto-ballistic effects in non-centrosymmetric GaAs/AlGaAs cross junctions

Magneto-ballistic transport is studied in a mesoscopic four-terminal junction with broken symmetry in the classical ballistic transport regime. Depending on the measurement configuration the device acts as a non-centrosymmetric Hall junction or a ballistic rectifier. In Hall configuration the absolute value of the Hall voltage depends on the current polarity. This nonlinearity is attributed to a current-polarity-dependent feeding of ballistic electrons into the Hall probes. A centrosymmetric reference sample shows the well-known linear Hall characteristic. In rectifier configuration the rectified inertial-ballistic signal is superimposed by the Hall component. The inertial-ballistic component is maintained in perpendicular magnetic fields exceeding B = 400 mT .

Hot-electron bend resistance in a ballistic GaAs/AlGaAs cross junction

Nonlinear bend resistance is studied in a six-terminal mesoscopic ballistic junction in the hot-electron regime. Non-equilibrium electrons are generated by a voltage drop across a quantum point contact (QPC) embedded in the current lead of an orthogonal cross junction. The barrier height of the QPC constriction is tuned by a large-area top gate electrode which covers junction and leads. The junction enables to study the ballistic bend voltage in local and remote configuration. In local configuration the ballistic voltage is detected between the voltage probes opposed to the orthogonal current leads. Independent from the current polarity a negative bend resistance verifies the ballistic motion of electrons. For small gate voltages, i.e. a high QPC barrier, the absolute value of the bend resistance is enhanced if the electrons are injected through the constriction. We attribute this nonlinearity to the current-polarity-dependent excess energy gained in the negatively biased current lead. If the transfer voltage is detected between two remote orthogonal voltage probes ballistic effects are observed only for small excess energies. The remote configuration implies that the electron ballistic mean-free path decreases at high excess energies.

Statistics and fluctuation theorem for boson and fermion transport through mesoscopic junctions

The statistical properties of quantum particles moving between two heat reservoirs at different temperatures are studied by solving the quantum master equation using a generating function technique. Bosons and fermions satisfy the same fluctuation theorem: Logarithm of the ratio of the probabilities of forward and backward transports reaches a constant value at long times, with an asymptotic ∼1t correction. Non-Poissonian transfer statistics (bunching for bosons and antibunching for fermions) are examined using the Mandel parameter. These come primarily from the tail of the distribution corresponding to transfer of large number of particles, k k. © 2007 The American Physical Society.