Andrei Kogan

Measurements of Kondo and spin splitting in single-electron transistors.

We measure the spin splitting in a magnetic field B of localized states in single-electron transistors using a new method, inelastic spin-flip cotunneling. Because it involves only internal excitations, this technique gives the most precise value of the Zeeman energy Delta=/g/mu(B)B. In the same devices we also measure the splitting with B of the Kondo peak in differential conductance. The Kondo splitting appears only above a threshold field as predicted by theory. However, the magnitude of the Kondo splitting at high fields exceeds 2/g/mu(B)B in disagreement with theory.

Room temperature photocurrent spectroscopy of single zincblende and wurtzite InP nanowires

The InP nanowires are grown by vapor-liquid-solid growth catalyzed by 20 and 50 nm gold nanoparticles on an InP substrate at 400 C using metal-organic chemical-vapor deposition. The 50 nm nanowires were grown with a V/III ratio of 350, while the 20 nm nanowires were grown at a V/III ratio of 700. 9 Extensive cw and time-resolved PL measurements at low temperatures have shown that the 20 nm wires are predominantly WZ, while the 50 nm nanowires are predominantly ZB, with some twins and stacking faults present as determined by high-resolution transmission electron microscopy imaging. 3,4,9 To obtain individual nano

Singlet–triplet transition in a single-electron transistor at zero magnetic field

We report sharp peaks in the differential conductance of a single-electron transistor (SET) at low temperature for gate voltages at which charge fluctuations are suppressed. For odd numbers of electrons we observe the expected Kondo peak at zero bias. For even numbers of electrons we generally observe Kondo-like features corresponding to excited states. For the latter, the excitation energy often decreases with gate voltage until a new zero-bias Kondo peak results. We ascribe this behavior to a singlet-triplet transition in zero magnetic field driven by the change of shape of the potential that confines the electrons in the SET.

Diabetes Alters Intracellular Calcium Transients in Cardiac Endothelial Cells

Diabetic cardiomyopathy (DCM) is a diabetic complication, which results in myocardial dysfunction independent of other etiological factors. Abnormal intracellular calcium ([Ca2+]i) homeostasis has been implicated in DCM and may precede clinical manifestation. Studies in cardiomyocytes have shown that diabetes results in impaired [Ca2+]i homeostasis due to altered sarcoplasmic reticulum Ca2+ ATPase (SERCA) and sodium-calcium exchanger (NCX) activity. Importantly, altered calcium homeostasis may also be involved in diabetes-associated endothelial dysfunction, including impaired endothelium-dependent relaxation and a diminished capacity to generate nitric oxide (NO), elevated cell adhesion molecules, and decreased angiogenic growth factors. However, the effect of diabetes on Ca2+ regulatory mechanisms in cardiac endothelial cells (CECs) remains unknown. The objective of this study was to determine the effect of diabetes on [Ca2+]i homeostasis in CECs in the rat model (streptozotocin-induced) of DCM. DCM-associated cardiac fibrosis was confirmed using picrosirius red staining of the myocardium. CECs isolated from the myocardium of diabetic and wild-type rats were loaded with Fura-2, and UTP-evoked [Ca2+]i transients were compared under various combinations of SERCA, sarcoplasmic reticulum Ca2+ ATPase (PMCA) and NCX inhibitors. Diabetes resulted in significant alterations in SERCA and NCX activities in CECs during [Ca2+]i sequestration and efflux, respectively, while no difference in PMCA activity between diabetic and wild-type cells was observed. These results improve our understanding of how diabetes affects calcium regulation in CECs, and may contribute to the development of new therapies for DCM treatment.

Regulation of endothelial MAPK/ ERK signalling and capillary morphogenesis by low-amplitude electric field

Low-amplitude electric field (EF) is an important component of wound-healing response and can promote vascular tissue repair; however, the mechanisms of action on endothelium remain unclear. We hypothesized that physiological amplitude EF regulates angiogenic response of microvascular endothelial cells via activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathway. A custom set-up allowed non-thermal application of EF of high (7.5 GHz) and low (60 Hz) frequency. Cell responses following up to 24 h of EF exposure, including proliferation and apoptosis, capillary morphogenesis, vascular endothelial growth factor (VEGF) expression and MAPK pathways activation were quantified. A db/db mouse model of diabetic wound healing was used for in vivo validation. High-frequency EF enhanced capillary morphogenesis, VEGF release, MEK-cRaf complex formation, MEK and ERK phosphorylation, whereas no MAPK/JNK and MAPK/p38 pathways activation was observed. The endothelial response to EF did not require VEGF binding to VEGFR2 receptor. EF-induced MEK phosphorylation was reversed in the presence of MEK and Ca2+ inhibitors, reduced by endothelial nitric oxide synthase inhibition, and did not depend on PI3K pathway activation. The results provide evidence for a novel intracellular mechanism for EF regulation of endothelial angiogenic response via frequency-sensitive MAPK/ERK pathway activation, with important implications for EF-based therapies for vascular tissue regeneration.

Kondo temperature dependence of the Kondo splitting in a single-electron transistor

A Kondo peak in the differential conductance of a single-electron transistor is measured as a function of both magnetic field and the Kondo temperature. We observe that the Kondo splitting decreases logarithmically with Kondo temperature and that there exists a critical magnetic field Bc below which the Kondo peak does not split, in qualitative agreement with theory. However, we find that the magnitude of the prefactor of the logarithm is larger than predicted and is independent of B, in contradiction with theory. Our measurements also suggest that the value of Bc is smaller than predicted.

Modulation of cell function by electric field: a high-resolution analysis.

Regulation of cell function by a non-thermal, physiological-level electromagnetic field has potential for vascular tissue healing therapies and advancing hybrid bioelectronic technology. We have recently demonstrated that a physiological electric field (EF) applied wirelessly can regulate intracellular signalling and cell function in a frequency-dependent manner. However, the mechanism for such regulation is not well understood. Here, we present a systematic numerical study of a cell-field interaction following cell exposure to the external EF. We use a realistic experimental environment that also recapitulates the absence of a direct electric contact between the field-sourcing electrodes and the cells or the culture medium. We identify characteristic regimes and present their classification with respect to frequency, location, and the electrical properties of the model components. The results show a striking difference in the frequency dependence of EF penetration and cell response between cells suspended in an electrolyte and cells attached to a substrate. The EF structure in the cell is strongly inhomogeneous and is sensitive to the physical properties of the cell and its environment. These findings provide insight into the mechanisms for frequency-dependent cell responses to EF that regulate cell function, which may have important implications for EF-based therapies and biotechnology development.

Spin polarization control by electric stirring: proposal for a spintronic device

We propose a spintronic device to generate spin polarization in a mesoscopic region by purely electric means. We show that the spin Hall effect in combination with the stirring effect are sufficient to induce measurable spin polarization in a closed geometry. Our device structure does not require the application of magnetic fields, external radiation or ferromagnetic leads, and can be implemented in standard semiconducting materials.

Dynamic response of a spin-12Kondo singlet

We present a study of spin 1/2 Kondo singlets in single electron transistors under a microwave frequency bias excitation. We compare time-averaged conductance

Magnetic-field-induced crossover to a nonuniversal regime in a Kondo dot.

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