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Biomedical subjects

M Pepper

Publications and source records attributed to M Pepper.

At least 19 recordsLinked to original sources
PubMed2006

Zeeman splitting in ballistic hole quantum wires.

We have studied the Zeeman splitting in ballistic hole quantum wires formed in a (311)A quantum well by surface gate confinement. Transport measurements clearly show lifting of the spin degeneracy and crossings of the subbands when an in-plane magnetic field B is applied parallel to the wire. When B is oriented perpendicular to the wire, no spin splitting is discernible up to B = 8.8 T. The observed large Zeeman splitting anisotropy in our hole quantum wires demonstrates the importance of quantum confinement for spin splitting in nanostructures with strong spin-orbit coupling.

Journal Article↗
PubMed2006

Conductance quantization at a half-integer plateau in a symmetric GaAs quantum wire.

We present data from an induced gallium arsenide (GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e2/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.

Journal Article↗
PubMed2005

Zero-bias anomaly and kondo-assisted quasiballistic 2D transport.

Nonequilibrium transport measurements in mesoscopic quasiballistic 2D electron systems show an enhancement in the differential conductance around the Fermi energy. At very low temperatures, such a zero-bias anomaly splits, leading to a suppression of linear transport at low energies. We also observed a scaling of the nonequilibrium characteristics at low energies which resembles electron scattering by two-state systems, addressed in the framework of two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an intrinsic phenomenon in unconfined 2D systems in the low electron-density regime.

Journal Article↗
PubMed2004

Fano factor reduction on the 0.7 conductance structure of a ballistic one-dimensional wire.

We have measured the nonequilibrium current noise in a ballistic one-dimensional wire which exhibits an additional conductance plateau at 0.7x2e(2)/h. The Fano factor shows a clear reduction on the 0.7 structure, and eventually vanishes upon applying a strong parallel magnetic field. These results provide experimental evidence that the 0.7 structure is associated with two conduction channels that have different transmission probabilities.

Journal Article↗
PubMed2004

In vivo study of human skin using pulsed terahertz radiation.

Studies in terahertz (THz) imaging have revealed a significant difference between skin cancer (basal cell carcinoma) and healthy tissue. Since water has strong absorptions at THz frequencies and tumours tend to have different water content from normal tissue, a likely contrast mechanism is variation in water content. Thus, we have previously devised a finite difference time-domain (FDTD) model which is able to closely simulate the interaction of THz radiation with water. In this work we investigate the interaction of THz radiation with normal human skin on the forearm and palm of the hand in vivo. We conduct the first ever systematic in vivo study of the response of THz radiation to normal skin. We take in vivo reflection measurements of normal skin on the forearm and palm of the hand of 20 volunteers. We compare individual examples of THz responses with the mean response for the areas of skin under investigation. Using the in vivo data, we demonstrate that the FDTD model can be applied to biological tissue. In particular, we successfully simulate the interaction of THz radiation with the volar forearm. Understanding the interaction of THz radiation with normal skin will form a step towards developing improved imaging algorithms for diagnostic detection of skin cancer and other tissue disorders using THz radiation.

Algorithms↗
PubMed2004

Possible evidence of a spontaneous spin polarization in mesoscopic two-dimensional electron systems.

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems.

Journal Article↗
PubMed2003

Interaction effects at crossings of spin-polarized one-dimensional subbands.

We report conductance measurements of ballistic one-dimensional (1D) wires defined in GaAs/AlGaAs heterostructures in an in-plane magnetic field, B. When the Zeeman energy is equal to the 1D subband energy spacing, the spin-split subband N upward arrow intersects (N+1) downward arrow, where N is the index of the spin-degenerate 1D subband. At the crossing of N=1 upward arrow and N=2 downward arrow subbands, there is a spontaneous splitting giving rise to an additional conductance structure evolving from the 1.5(2e(2)/h) plateau. With further increase in B, the structure develops into a plateau and lowers to 2e(2)/h. With increasing temperature and magnetic field the structure shows characteristics of the 0.7 structure. Our results suggest that at low densities a spontaneous spin splitting occurs whenever two 1D subbands of opposite spins cross.

Journal Article↗
PubMed2003

Using Terahertz pulse spectroscopy to study the crystalline structure of a drug: a case study of the polymorphs of ranitidine hydrochloride.

We describe the application of Terahertz pulse spectroscopy to polymorph identification. The particular compounds investigated were the different crystalline Forms 1 and 2 of ranitidine hydrochloride, both in the pure form and also obtained as a marketed pharmaceutical product. Identification was clear. The technique has advantages that excitation is not via a powerful laser source, as used in Raman spectroscopy, so phase changes or photochemical reactions in polymorphs do not occur. Terahertz absorption spectral interpretation and instrumentation are similar to basic Fourier transform infrared (FTIR) spectroscopy and therefore easy to understand. The sample preparation techniques used are the same as those used in FTIR and Raman spectroscopies. The data obtained is complementary to Raman Spectroscopy. As the selection rules are different between the two techniques, we are able to obtain new data set directly related to crystalline structure adding to that obtained by Raman spectroscopy. Terahertz pulse spectroscopy provides information on low-frequency intermolecular vibrational modes; these are difficult to assess in Raman spectroscopy due to the proximity of the laser exciting line. It is concluded that the method has a wide range of applications in pharmaceutical science including formulation, high throughput screening, and inspection in storage.

Chemistry, Pharmaceutical↗
PubMed2003

Dielectrophoretic manipulation of surface-bound DNA.

Dielectrophoretic manipulation enables the positioning and orientation of DNA molecules for nanometer-scale applications. However, the dependence of the dielectrophoretic force and torque on the electric field magnitude and frequency has to be well characterised to realise fully the potential of this technique. DNA in solution is attracted to the strongest electric field gradient (i.e. the electrode edge) as a result of the dielectrophoretic force, while the dielectrophoretic torque aligns the DNA with its longest axis parallel to the electric field. In this work, the authors attached -DNA fragments (48 and 25 kilobases) to an array of gold microelectrodes via a terminal thiol bond and characterised the orientation and elongation as a function of electric field magnitude (0.1-0.8 MVm) and frequency (0.08-1.1 MHz). Maximum elongation was observed between 200 and 500 kHz for the attached DNA. Dielectrophoresis is limited by thermal randomisation at electric fields below 0.1 MVm and by electrothermal effects above 0.7 MVm. The authors conclude that dielectrophoresis can be used to manipulate surface-immobilised DNA reproducibly.

Journal Article↗
PubMed2002

Origin of the oscillator strength of the triplet state of a trion in a magnetic field.

The dynamics of the spin-triplet trion state, under high magnetic field in a GaAs/AlGaAs quantum well, are studied using time resolved spectroscopy. The oscillator strength of the triplet transition is shown to rise with increasing electron density, in good agreement with a theoretical model where the trion interacts with excess electrons in the quantum well. This analysis suggests that the spin-triplet trion state, which is expected to be an optically "dark" state, is experimentally observable due to the interactions with the excess electrons, demonstrating that X- cannot be regarded as an isolated three particle complex.

Journal Article↗
PubMed2002

Hole-hole interaction effect in the conductance of the two-dimensional hole gas in the ballistic regime.

On a high-mobility two-dimensional hole gas (2DHG) in a GaAs/GaAlAs heterostructure we study the interaction correction to the Drude conductivity in the ballistic regime, k(B)Ttau/ variant Planck's over 2pi >1. It is shown that the "metallic" behavior of the resistivity (drho/dT>0) of the low-density 2DHG is caused by the hole-hole interaction effect in this regime. We find that the temperature dependence of the conductivity and the parallel-field magnetoresistance are in agreement with this description, and determine the Fermi-liquid interaction constant Fsigma0 which controls the sign of drho/dT.

Journal Article↗
PubMed2001

Observation of charge transport by negatively charged excitons.

We report transport of electron-hole complexes in semiconductor quantum wells under applied electric fields. Negatively charged excitons (X-), created by laser excitation of a high electron mobility transistor, are observed to drift upon applying a voltage between the source and drain. In contrast, neutral excitons do not drift under similar conditions. The X- mobility is found to be as high as 6.5 x 10(4) cm2 V-1 s-1. The results demonstrate that X- exists as a free particle in the best-quality samples and suggest that light emission from opto-electronic devices can be manipulated through exciton drift under applied electric fields.

Journal Article↗
PubMed2001

Metallic behavior in dilute two-dimensional hole systems.

We have studied the metallic behavior in low-density two-dimensional p-GaAs systems, close to the apparent metal-insulator transition. Two observations are made concerning the origins of the metallic-like behavior. Within a given sample the strength of the metallic behavior is almost independent of the asymmetry of the confining potential, and is predominantly determined by the low-temperature resistivity (i.e., by k(F)l). In all our samples we find that at low densities, close to the transition from insulating to metallic behavior, the fractional decrease in conductivity with increasing temperature scales as T/T(F).

Journal Article↗
PubMed2001

Fermi-liquid behavior of the low-density 2D hole gas in a GaAs/AlGaAs heterostructure at large values of r(s).

We examine the validity of the Fermi-liquid description of the dilute 2D hole gas in the crossover from "metallic"-to-"insulating" behavior of rho(T). It has been established that, at r(s) as large as 29, negative magnetoresistance does exist and is well described by weak localization theory. The dephasing time, extracted from the magnetoresistance, is dominated by the T2 term due to hole-hole scattering in the clean limit. The effect of hole-hole interactions, however, is suppressed when compared with the theory derived for small r(s).

Journal Article↗
PubMed2001

Continuous insulin infusion: promoting growth in low birth weight infants.

Development of improved technologies in neonatal care has yielded increasing numbers of surviving low birth weight (LBW) infants who have challenged methods of supportive care. As researchers and practitioners have focused on respiratory, cardiac, and other body system requirements, nutritional support has been low on the priority list. Support for growth and maximized neurologic development, rather than simply for survival, has been the challenge in care of small and sick newborns. Use of insulin to enhance glucose tolerance in LBW infants, and thereby maximize growth, is a management modality that can be initiated early in the infant's course and is facilitated by implementation of clear and consistent policies and procedures. Continuous insulin infusions have been demonstrated to enhance glucose uptake and utilization--facilitating neonatal growth and in turn enhancing brain growth and developmental outcome.

Drug Monitoring↗
PubMed2000

Weak localization, hole-hole interactions, and the "Metal"-insulator transition in two dimensions

A detailed investigation of the metallic behavior in high-quality GaAs-AlGaAs two-dimensional hole systems reveals the presence of quantum corrections to the resistivity at low temperatures. Despite the low density ( r(s)>10) and high quality of these systems, both weak localization (observed via negative magnetoresistance) and weak hole-hole interactions (giving a correction to the Hall constant) are present in the so-called metallic phase where the resistivity decreases with decreasing temperature. If these quantum corrections persist down to T = 0, the results suggest that even at high r(s) there is no metallic phase in two dimensions.

Journal Article↗