Local moment formation in quantum point contacts

Spin-density-functional theory of quantum point contacts (QPCs) reveals the formation of a local moment with a net of one electron spin in the vicinity of the point contact - supporting the recent report of a Kondo effect in a QPC. The hybridization of the local moment to the leads decreases as the QPC becomes longer, while the onsite Coulomb-interaction energy remains almost constant.Comment: 10 pages, 3 figures, accepted for publication in Physical Review Letter

Kondo model for the "0.7 anomaly" in transport through a quantum point contact

Experiments on quantum point contacts have highlighted an anomalous conductance plateau at $0.7 (2e^2/h)$, with features suggestive of the Kondo effect. Here we present an Anderson model for transport through a point contact which we analyze in the Kondo limit. Hybridization to the band increases abruptly with energy but decreases with valence, so that the background conductance and the Kondo temperature $T_K$ are dominated by different valence transitions. This accounts for the high residual conductance above $T_K$. A spin-polarized current is predicted for Zeeman splitting $g^* \mu_B B > k_B T_K,k_BT$.Comment: 4 page

Conductance anomalies and the extended Anderson model for nearly perfect quantum wires

Anomalies near the conductance threshold of nearly perfect semiconductor quantum wires are explained in terms of singlet and triplet resonances of conduction electrons with a single weakly-bound electron in the wire. This is shown to be a universal effect for a wide range of situations in which the effective single-electron confinement is weak. The robustness of this generic behavior is investigated numerically for a wide range of shapes and sizes of cylindrical wires with a bulge. The dependence on gate voltage, source-drain voltage and magnetic field is discussed within the framework of an extended Hubbard model. This model is mapped onto an extended Anderson model, which in the limit of low temperatures is expected to lead to Kondo resonance physics and pronounced many-body effects

Extreme sensitivity of the spin-splitting and 0.7 anomaly to confining potential in one-dimensional nanoelectronic devices

Quantum point contacts (QPCs) have shown promise as nanoscale spin-selective components for spintronic applications and are of fundamental interest in the study of electron many-body effects such as the 0.7 x 2e^2/h anomaly. We report on the dependence of the 1D Lande g-factor g* and 0.7 anomaly on electron density and confinement in QPCs with two different top-gate architectures. We obtain g* values up to 2.8 for the lowest 1D subband, significantly exceeding previous in-plane g-factor values in AlGaAs/GaAs QPCs, and approaching that in InGaAs/InP QPCs. We show that g* is highly sensitive to confinement potential, particularly for the lowest 1D subband. This suggests careful management of the QPC's confinement potential may enable the high g* desirable for spintronic applications without resorting to narrow-gap materials such as InAs or InSb. The 0.7 anomaly and zero-bias peak are also highly sensitive to confining potential, explaining the conflicting density dependencies of the 0.7 anomaly in the literature.Comment: 23 pages, 7 figure

The Low-Temperature Fate of the 0.7 Structure in a Point Contact: A Kondo-like Correlated State in an Open System

Besides the usual conductance plateaus at multiples of 2e2/h, quantum point contacts typically show an extra plateau at ~ 0.7(2e2/h), believed to arise from electron-electron interactions that prohibit the two spin channels from being simultaneously occupied. We present evidence that the disappearance of the 0.7 structure at very low temperature signals the formation of a Kondo-like correlated spin state. Evidence includes a zero-bias conductance peak that splits in a parallel field, scaling of conductance to a modified Kondo form, and consistency between peak width and the Kondo temperature

Cryogenic Investigation of Current Collapse in AlGaN/GaN HFETS

Current collapse in AlGaN/GaN HFETs is investigated at low temperatures using a transient current monitoring technique. The carrier trapping and de-trapping mechanisms are studied, and two distinct relaxation mechanisms are observed. They are associated to the presence of two close deep energy levels in the bandgap

Temperature-Dependent Analysis and RF-Model of 10Gbps VCSELs

10Gbps Vertical Cavity Surface Emitting Lasers (VCSELs) are fully characterized and modeled at various temperatures of operation from DC to 15GHz. Studying devices under these conditions of operation enables one to acquire a better understanding of the device physics, and permits to build temperature-dependent VCSEL RF models necessary for accurate opto-electronic integrated circuit (OEIC) designs. The extracted model accurately predicts the reflection coefficient up to 15GHz above and below threshold at various biases and temperatures. This is the first time that a temperature-dependent RF model for VCSEL is presented

Floating-Body Effects in AlGaN/GaN Power HFETs

AlGaN/GaN power HFETs grown on a 200nm thick AlN sub-buffer layer are investigated. The presence of a steady kink in the static and dynamic drain-to-source current characteristics is attributed to floating-body (FB) effects that result from the AlN sub-buffer layer. A method to extract the off-state body-to-source voltage (VBS) is applied and the coherence of the results confirms that FB effects are present in this type of structures. To our knowledge, this is the first time that floating-body effects are reported and modeled in AlGaN/GaN HFETs

Noise, Large-Signal Modeling and Characterization of InP/InGaAs HBTs

We developed a robust large-signal model for InP/InGaAs HBTs. DC, small-signal, noise and power characteristics of InP/InGaAs HBTs are measured over a wide range of frequencies and bias conditions. A minimum noise figure (FMIN) of 3.5dB, and a gain of 16.8dB are achieved at 10-GHz. These measurement results are the basis for robust nonlinear models of InP/InGaAs HBT devices