Manan Mehta

Coexistence of superconductivity and ferromagnetism in two dimensions.

Ferromagnetism is usually considered to be incompatible with conventional superconductivity, as it destroys the singlet correlations responsible for the pairing interaction. Superconductivity and ferromagnetism are known to coexist in only a few bulk rare-earth materials. Here we report evidence for their coexistence in a two-dimensional system: the interface between two bulk insulators, LaAlO(3) (LAO) and SrTiO(3) (STO), a system that has been studied intensively recently. Magnetoresistance, Hall, and electric-field dependence measurements suggest that there are two distinct bands of charge carriers that contribute to the interface conductivity. The sensitivity of properties of the interface to an electric field makes this a fascinating system for the study of the interplay between superconductivity and magnetism.

TCA Cycle and Mitochondrial Membrane Potential Are Necessary for Diverse Biological Functions

Mitochondrial metabolism is necessary for the maintenance of oxidative TCA cycle function and mitochondrial membrane potential. Previous attempts to decipher whether mitochondria are necessary for biological outcomes have been hampered by genetic and pharmacologic methods that simultaneously disrupt multiple functions linked to mitochondrial metabolism. Here, we report that inducible depletion of mitochondrial DNA (ρ(ο) cells) diminished respiration, oxidative TCA cycle function, and the mitochondrial membrane potential, resulting in diminished cell proliferation, hypoxic activation of HIF-1, and specific histone acetylation marks. Genetic reconstitution only of the oxidative TCA cycle function specifically in these inducible ρ(ο) cells restored metabolites, resulting in re-establishment of histone acetylation. In contrast, genetic reconstitution of the mitochondrial membrane potential restored ROS, which were necessary for hypoxic activation of HIF-1 and cell proliferation. These results indicate that distinct mitochondrial functions associated with respiration are necessary for cell proliferation, epigenetics, and HIF-1 activation.

Mitochondrial control of immunity: beyond ATP

Mitochondria are important signalling organelles, and they dictate immunological fate. From T cells to macrophages, mitochondria form the nexus of the various metabolic pathways that define each immune cell subset. In this central position, mitochondria help to control the various metabolic decision points that determine immune cell function. In this Review, we discuss how mitochondrial metabolism varies across different immune cell subsets, how metabolic signalling dictates cell fate and how this signalling could potentially be targeted therapeutically.

Evidence for charge–vortex duality at the LaAlO3/SrTiO3 interface

The concept of duality has proved extremely powerful in extending our understanding in many areas of physics. Charge-vortex duality has been proposed as a model to understand the superconductor to insulator transition in disordered thin films and Josephson junction arrays. In this model, on the superconducting side, one has delocalized Cooper pairs but localized vortices; while on the insulating side, one has localized Cooper pairs but mobile vortices. Here we show a new experimental manifestation of this duality in the electron gas that forms at the interface between LaAlO(3) and SrTiO(3). The effect is due to the motion of vortices generated by the magnetization dynamics of the ferromagnet that also forms at the same interface, which results in an increase in resistance on the superconducting side of the transition, but an increase in conductance on the insulating side.

Phase-incoherent superconducting pairs in the normal state of Ba(Fe1-xCox)2As2

The normal state properties of the recently discovered ferropnictide superconductors might hold the key to understanding their exotic superconductivity. Using point-contact spectroscopy we show that Andreev reflection between an epitaxial thin film of Ba(Fe(0.92)Co(0.08))₂As₂ and a silver tip can be seen in the normal state of the film up to temperature T∼1.3T(c), where T(c) is the critical temperature of the superconductor. Andreev reflection far above T(c) can be understood only when superconducting pairs arising from strong fluctuation of the phase of the complex superconducting order parameter exist in the normal state. Our results provide spectroscopic evidence of phase-incoherent superconducting pairs in the normal state of the ferropnictide superconductors.

Conductance asymmetry in point-contacts on epitaxial thin films of Ba(Fe0.92Co0.08)2As2

One of the most common observations in point-contact spectra on the recently discovered ferropnictide superconductors is a large conductance asymmetry with respect to voltage across the point-contact. In this paper, we show that the antisymmetric part of the point-contact spectrum between a silver tip and an epitaxial thin film of Ba(Fe0.92Co0.08)2As2 shows certain unique features that have an interesting evolution with increasing temperature up to a temperature far above the critical temperature Tc. We associate this observation with the existence of a gap above Tc that might originate from strong fluctuations of the phase of the superconducting order parameter.

Gate-tunable superconducting weak link behavior in top-gated LaAlO3-SrTiO3

We use a combination of global back-gating and local top-gating to define nanoscale devices in the two-dimensional electron gas at the LaAlO3-SrTiO3 interface, demonstrating an efficient way for much finer spatial control over the properties of the interface, as compared to back-gating alone. The devices show indications of an inhomogenous superconducting weak link. The variation of critical current with perpendicular magnetic field shows evidence of oscillations, which hints at Josephson coupling. The variation of the critical current and zero bias resistance with temperature is consistent with short, overdamped weak links. We show that the applied top-gate voltage provides a strong handle on the properties of these weak links. This technique can be an important tool to define a variety of device structures in this system, allowing us to probe the nature of superconductivity in the LaAlO3-SrTiO3 interface system in different ways.

RTSPM: Real-time Linux control software for scanning probe microscopy

Real time computer control is an essential feature of scanning probe microscopes, which have become important tools for the characterization and investigation of nanometer scale samples. Most commercial (and some open-source) scanning probe data acquisition software uses digital signal processors to handle the real time data processing and control, which adds to the expense and complexity of the control software. We describe here scan control software that uses a single computer and a data acquisition card to acquire scan data. The computer runs an open-source real time Linux kernel, which permits fast acquisition and control while maintaining a responsive graphical user interface. Images from a simulated tuning-fork based microscope as well as a standard topographical sample are also presented, showing some of the capabilities of the software.

Targeting metabolism for lupus therapy

Drugs that correct defects in T cell metabolism may prove to be promising treatments for lupus. Drugs that correct defects in T cell metabolism may prove to be promising treatments for lupus.

A hybrid analog-digital phase-locked loop for frequency mode non-contact scanning probe microscopy

Non-contact scanning probe microscopy (SPM) has developed into a powerful technique to image many different properties of samples. The conventional method involves monitoring the amplitude, phase, or frequency of a cantilever oscillating at or near its resonant frequency as it is scanned across the surface of a sample. For high Q factor cantilevers, monitoring the resonant frequency is the preferred method in order to obtain reasonable scan times. This can be done by using a phase-locked-loop (PLL). PLLs can be obtained as commercial integrated circuits, but these do not have the frequency resolution required for SPM. To increase the resolution, all-digital PLLs requiring sophisticated digital signal processors or field programmable gate arrays have also been implemented. We describe here a hybrid analog/digital PLL where most of the components are implemented using discrete analog integrated circuits, but the frequency resolution is provided by a direct digital synthesis chip controlled by a simple peripheral interface controller (PIC) microcontroller. The PLL has excellent frequency resolution and noise, and can be controlled and read by a computer via a universal serial bus connection.