M. Henini

Real‐time scanning Hall probe microscopy

We describe a low‐noise scanning Hall probe microscope having unprecedented magnetic field sensitivity (∼2.9×10−8 T/√Hz at 77 K), high spatial resolution, (∼0.85 μm), and operating in real‐time (∼1 frame/s) for studying flux profiles at surfaces. A submicron Hall probe manufactured in a GaAs/AlGaAs two‐dimensional electron gas (2DEG) is scanned over the sample to measure the surface magnetic fields using conventional scanning tunneling microscopy positioning techniques. Flux penetration into a high Tc YBa2Cu3O7−δ thin film has been observed in real time at 85 K with single vortex resolution. Flux is seen to enter the film in the form of vortex bundles as well as single flux quanta, Φ0.

Magnetoresistance of a two-dimensional electron gas due to a single magnetic barrier and its use for nanomagnetometry

We investigate the longitudinal resistance of a semiconductor near-surface two-dimensional electron gas (2DEG) subjected to a magnetic barrier induced by the stray field from a single sub-micron ferromagnetic line on the surface of the device. The amplitude of the magnetic barrier is controlled by the application of an external magnetic field in the plane of the 2DEG. We show that this type of magnetoresistance can be used to deduce properties of the ferromagnetic line, so that our hybrid ferromagnet-semiconductor structure acts as a nanomagnetometer.

Paramagnetic Meissner effect in small superconductors

A superconductor placed in a magnetic field and cooled down through the transition temperature expels magnetic flux. This phenomenon, known as the Meissner effect, is arguably the most essential property of superconductors and implies zero resistivity. Surprisingly, several recent experiments have shown that some superconducting samples may attract magnetic field—the so-called paramagnetic Meissner effect. The scarce, if not controversial, experimental evidence for this effect makes it difficult to identify the origin of this enigmatic phenomenon, although a large number of possible explanations have been advanced. Here we report observations of the paramagnetic Meissner effect with a resolution better than one quantum of magnetic flux. The paramagnetic Meissner effect is found to be an oscillating function of the magnetic field (due to flux quantization) and replaces the normal Meissner effect only above a certain field when several flux quanta are frozen inside a superconductor. The paramagnetic state is found to be metastable and the Meissner state can be restored by external noise. We conclude that the paramagnetic Meissner effect is related to the surface superconductivity and, therefore, represents a general property of superconductors: on decreasing temperature, the flux captured at the third (surface) critical field inside the superconducting sheath compresses into a smaller volume, allowing extra flux to penetrate at the surface.

High temperature gate control of quantum well spin memory.

Time-resolved optical measurements in (110)-oriented GaAs/AlGaAs quantum wells show a tenfold increase of the spin-relaxation rate as a function of applied electric field from 20 to 80 kV cm(-1) at 170 K and indicate a similar variation at 300 K, in agreement with calculations based on the Rashba effect. Spin relaxation is almost field independent below 20 kV cm(-1) reflecting quantum well interface asymmetry. The results indicate the achievability of a voltage-gateable spin-memory time longer than 3 ns simultaneously with a high electron mobility.

Photodegradation of organic pollutants RhB dye using UV simulated sunlight on ceria based TiO 2 nanomaterials for antibacterial applications

To photo-catalytically degrade RhB dye using solar irradiation, CeO2 doped TiO2 nanocomposites were synthesized hydrothermally at 700 °C for 9 hrs. All emission spectra showed a prominent band centered at 442 nm that was attributed to oxygen related defects in the CeO2-TiO2 nanocrystals. Two sharp absorption bands at 1418 cm−1 and 3323 cm−1 were attributed to the deformation and stretching vibration, and bending vibration of the OH group of water physisorbed to TiO2, respectively. The photocatalytic activities of Ce-TiO2 nanocrystals were investigated through the degradation of RhB under UV and UV+ visible light over a period of 8 hrs. After 8 hrs, the most intense absorption peak at 579 nm disappeared under the highest photocatalytic activity and 99.89% of RhB degraded under solar irradiation. Visible light-activated TiO2 could be prepared from metal-ion incorporation, reduction of TiO2, non-metal doping or sensitizing of TiO2 using dyes. Studying the antibacterial activity of Ce-TiO2 nanocrystals against E. coli revealed significant activity when 10 μg was used, suggesting that it can be used as an antibacterial agent. Its effectiveness is likely related to its strong oxidation activity and superhydrophilicity. This study also discusses the mechanism of heterogeneous photocatalysis in the presence of TiO2.

Electronic structure of self-assembled InAs quantum dots in GaAs matrix

Capacitance–voltage characteristics have been measured at various frequencies and temperatures for structures containing a sheet of self-assembled InAs quantum dots in both n-GaAs and p-GaAs matrices. Analysis of the capacitance–voltage characteristics shows that the deposition of 1.7 ML of InAs forms quantum dots with electron levels 80 meV below the bottom of the GaAs conduction band and two heavy-hole levels at 100 and 170 meV above the top of the GaAs valence band. The carrier energy levels agree very well with the recombination energies obtained from photoluminescence spectra.

Chaotic electron diffusion through stochastic webs enhances current flow in superlattices.

Understanding how complex systems respond to change is of fundamental importance in the natural sciences. There is particular interest in systems whose classical newtonian motion becomes chaotic as an applied perturbation grows. The transition to chaos usually occurs by the gradual destruction of stable orbits in parameter space, in accordance with the Kolmogorov–Arnold–Moser (KAM) theorem—a cornerstone of nonlinear dynamics that explains, for example, gaps in the asteroid belt. By contrast, ‘non-KAM’ chaos switches on and off abruptly at critical values of the perturbation frequency. This type of dynamics has wide-ranging implications in the theory of plasma physics, tokamak fusion, turbulence, ion traps, and quasicrystals. Here we realize non-KAM chaos experimentally by exploiting the quantum properties of electrons in the periodic potential of a semiconductor superlattice with an applied voltage and magnetic field. The onset of chaos at discrete voltages is observed as a large increase in the current flow due to the creation of unbound electron orbits, which propagate through intricate web patterns in phase space. Non-KAM chaos therefore provides a mechanism for controlling the electrical conductivity of a condensed matter device: its extreme sensitivity could find applications in quantum electronics and photonics.

Temperature dependence of the photoluminescence emission from thiol-capped PbS quantum dots

The authors report the temperature dependence of the near-infrared photoluminescence (PL) emission from thiol-capped PbS quantum dots. The high thermal stability of the PL allows the authors to study the thermal broadening of the dot emission over an extended temperature range (4–300K). The authors show that the linewidth of the dot PL emission is strongly enhanced at temperatures above 150K. This behavior is attributed to dephasing of the quantum electronic states by carrier interaction with longitudinal optical phonons. The authors’ data also indicate that the strength of the carrier-phonon coupling is larger in smaller dots.

Scanning Hall probe microscopy of superconductors and magnetic materials

We describe results from a scanning Hall probe microscope operating in a broad temperature range, 4–300 K. A submicron Hall probe manufactured in a GaAs/AlGaAs two‐dimensional electron gas is scanned over the sample to measure the surface magnetic fields using conventional scanning tunneling microscopy positioning techniques. The magnetic field structure of the sample together with the topography can be obtained simultaneously. The technique is noninvasive with an extremely low self‐field of <10−2 G and yields a quantitative measurement of the surface magnetic field in contrast to magnetic force microscopy. In addition the microscope has an outstanding magnetic field resolution (∼1.1×10−3 G/√Hz at 77 K) and high spatial resolution, ∼0.85 μm. Images of individual vortices in a high‐Tc Y1Ba2Cu3O7−δ thin film at low temperatures and magnetic domains in an Fe‐garnet crystal at room temperature are presented.

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.