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P. T. Coleridge

Publications and source records attributed to P. T. Coleridge.

16 recordsLinked to original sources
arXiv2005

Electron spin-orbit splitting in InGaAs/InP quantum well studied by means of the weak antilocalization and spin-zero effects in tilted magnetic fields

The coupling between Zeeman spin splitting and Rashba spin-orbit terms has been studied experimentally in a gated InGaAs/InP quantum well structure by means of simultaneous measurements of the weak antilocalization (WAL) effect and beating in the SdH oscillations. The strength of the Zeeman splitting was regulated by tilting the magnetic field with the spin-zeros in the SdH oscillations, which are not always present, being enhanced by the tilt. In tilted fields the spin-orbit and Zeeman splittings are not additive, and a simple expression is given for the energy levels. The Rashba parameter and the electron g-factor were extracted from the position of the spin zeros in tilted fields. A good agreement is obtained for the spin-orbit coupling strength from the spin-zeros and weak antilocalization measurements.

cond-mat.mes-hall↗
arXiv2003

Weak antilocalization in a strained InGaAs/InP quantum well structure

Weak antilocalization (WAL) effect due to the interference corrections to the conductivity has been studied experimentally in a strained InGaAs/InP quantum well structure. From measurements in tilted magnetic filed, it was shown that both weak localization and WAL features depend only on the normal component of the magnetic field for tilt angles less than 84 degrees. Weak antilocalization effect showed non-monotonous dependence on the gate voltage which could not be explained by either Rashba or Dresselhouse mechanisms of the spin-orbit coupling. To describe magnetic field dependence of the conductivity, it was necessary to assume that spin-orbit scattering time depends on the external magnetic field which quenches the spin precession around effective, spin-orbit related, magnetic fields.

cond-mat.mes-hall↗
arXiv2003

Experimental study of weak antilocalization effect in a high mobility InGaAs/InP quantum well

The magnetoresistance associated with quantum interference corrections in a high mobility, gated InGaAs/InP quantum well structure is studied as a function of temperature, gate voltage, and angle of the tilted magnetic field. Particular attention is paid to the experimental extraction of phase-breaking and spin-orbit scattering times when weak anti- localization effects are prominent. Compared with metals and low mobility semiconductors the characteristic magnetic field $B_{tr} = \hbar/4eD τ$ in high mobility samples is very small and the experimental dependencies of the interference effects extend to fields several hundreds of times larger. Fitting experimental results under these conditions therefore requires theories valid for arbitrary magnetic field. It was found, however, that such a theory was unable to fit the experimental data without introducing an extra, empirical, scale factor of about 2. Measurements in tilted magnetic fields and as a function of temperature established that both the weak localization and the weak anti-localization effects have the same, orbital origin. Fits to the data confirmed that the width of the low field feature, whether a weak localization or a weak anti-localization peak, is determined by the phase-breaking time and also established that the universal (negative) magnetoresistance observed in the high field limit is associated with a temperature independent spin-orbit scattering time.

cond-mat.mes-hall↗
arXiv2003

Microwave radiation induced magneto-oscillations in the longitudinal and transverse resistance of a two dimensional electron gas

We confirm the existance of magneto-resistance oscillations in a microwave-irradiated two-dimensional electron gas, first reported in a series of papers by Zhudov et al. and Mani et al. In our experiments, on a sample with a more moderate mobility, the microwave induced oscillations are observed not only in the longitudinal - but also in the transverse-resistance (Hall resistance). The phase of the oscillations is such that the decrease (increase) in the longitudinal resistance is accompanied by an increase (decrease) in the absolute value of the Hall resistance. We believe that these new results provide valuable new information to better understand the origin of this interesting phenomenon.

cond-mat.mes-hall↗
arXiv2003

Anomalous spin-orbit effects in a strained InGaAs/InP quantum well structure

There currently is a large effort to explore spin-orbit effects in semiconductor structures with the ultimate goal of manipulating electron spins with gates. A search for materials with large spin-orbit coupling is therefore important. We report results of a study of spin-orbit effects in a strained InGaAs/InP quantum well. The spin-orbit relaxation time, determined from the weak antilocalization effect, was found to depend non-monotonically on gate voltage. The spin orbit scattering rate had a maximum value of $5\times 10^{10}s^{-1}$ at an electron density of $n=3\times 10^{15} m^{-2}$. The scattering rate decreased from this for both increasing and decreasing densities. The smallest measured value was approximately $10^9 s^{-1}$ at an electron concentration of $n=6\times 10^{15} m^{-2}$. This behavior could not be explained by either the Rashba nor the bulk Dresselhaus mechanisms but is attributed to asymmetry or strain effects at dissimilar quantum well interfaces.

cond-mat.mes-hall↗
arXiv2002

Metallic and Insulating behaviour in p-SiGe at nu = 3/2

Shubnikov-de Haas data is presented for a p-SiGe sample exhibiting strongly insulating behaviour at nu = 3/2. In addition to the fixed points defining a high field metal- insulator transition into this phase separate fixed points can also be identified for the nu = 3 --> 2 and 2 --> 1 Integer quantum Hall transitions. Another feature of the data, that the Hall resistivity approaches zero in the insulating phase, indicates it is not a re-entrant Hall insulator. The behaviour is explained in terms of the strong exchange interactions. At integer filling factors these cause the 0 uparrow and 1 downarrow Landau levels to cross and be well separated but at non-integer values of nu screening reduces exchange effects and causes the levels to stick together. It is suggested the insulating behaviour, and high field metal/insulator transition, is a consequence of the strong exchange interactions.

cond-mat.mes-hall↗
arXiv2001

Weak localisation, interaction effects and the metallic phase in p-SiGe

Magnetoresistance results are presented for p-SiGe samples on the metallic side of the B=0 metal-insulator transition. It was possible to separate the weak localisation and Zeeman interaction effects but the results could not be explained quantitatively within the framework of standard theories for quantum corrections of a weakly interacting 2-dimensional system. Analysis using a theory for interaction corrections at intermediate temperatures, recently proposed by Zala, Narozhny and Aleiner, provided values of the Fermi liquid parameter $F_0^σ$ of order -0.5. Similar values also explain the linear increase of resistance with temperature characteristic of the metallic phase. e

cond-mat.str-el↗
arXiv2001

"Forbidden" transitions between quantum Hall and insulating phases in p-SiGe heterostructures

We show that in dilute metallic p-SiGe heterostructures, magnetic field can cause multiple quantum Hall-insulator-quantum Hall transitions. The insulating states are observed between quantum Hall states with filling factors ν=1 and 2 and, for the first time, between ν=2 and 3 and between ν=4 and 6. The latter are in contradiction with the original global phase diagram for the quantum Hall effect. We suggest that the application of a (perpendicular) magnetic field induces insulating behavior in metallic p-SiGe heterostructures in the same way as in Si MOSFETs. This insulator is then in competition with, and interrupted by, integer quantum Hall states leading to the multiple re-entrant transitions. The phase diagram which accounts for these transition is similar to that previously obtained in Si MOSFETs thus confirming its universal character.

cond-mat.str-el↗
arXiv2000

Interaction effects and the metallic phase in p-SiGe

Magnetoresistance results are presented for p-SiGe samples on the metallic side of the B=0 metal-insulator transition. The results cannot be understood within the framework of standard theories for quantum corrections of a weakly interacting 2- dimensional system. In particular no logarithmic dependence on temperature is observed, at low fields, in either the longitudinal or Hall resistivities despite evidence in the magnetoresistance of weak localisation effects. Further, the Hall coefficient shows a strong logarithmic dependence on field. The results are better explain by renormalisation group theories and by an anomalous Hall effect associated with strong spin-orbit coupling in the presence of a background spin texture.

cond-mat.str-el↗
arXiv1999

Quantum Phase Transitions in the p-SiGe system

The rich variety of phase transitions observed in the strained p-SiGe system are considered and compared. It is shown that the integer quantum Hall effect transitions, the Hall insulating transition and the re-entrant transition into an insulating phase near filling factor 3/2 are all very similar and good examples of quantum critical phase transitions. The B=0 metal insulator transition also shows many similarities to these transitions but requires the inclusion of an extra impurity scattering term to fully explain the data.

cond-mat.mes-hall↗
arXiv1999

Universal Behaviour of Metal-Insulator Transitions in the p-SiGe System

Magnetoresistance measurements are presented for a strained p-SiGe quantum well sample where the density is varied through the B=0 metal-insulator transition. The close relationship between this transition, the high field Hall insulator transition and the filling factor $ν$=3/2 insulating state is demonstrated.

cond-mat.mes-hall↗
arXiv1999

On the thermal broadening of a quantum critical phase transition

The temperature dependence of an integer Quantum Hall effect transition is studied in a sample where the disorder is dominated by short-ranged potential scattering. At low temperatures the results are consistent with a $(T/T_0)^κ$ scaling behaviour and at higher temperatures by a linear dependence similar to that reported in other material systems. It is shown that the linear behaviour results from thermal broadening produced by the Fermi-Dirac distribution function and that the temperature dependence over the whole range depends only on the scaling parameter T$_0^κ$.

cond-mat.mes-hall↗
arXiv1999

Universality in an integer Quantum Hall transition

An integer Quantum Hall effect transition is studied in a modulation doped p-SiGe sample. In contrast to most examples of such transitions the longitudinal and Hall conductivities at the critical point are close to 0.5 and 1.5 (e^2/h), the theoretically expected values. This allows the extraction of a scattering parameter, describing both conductivity components, which depends exponentially on filling factor. The strong similarity of this functional form to those observed for transitions into the Hall insulating state and for the B=0 metal- insulator transition implies a universal quantum critical behaviour for the transitions. The observation of this behaviour in the integer Quantum Hall effect, for this particular sample, is attributed to the short-ranged character of the potential associated with the dominant scatterers.

cond-mat.mes-hall↗
arXiv1997

Metal Insulator transition at B=0 in p-SiGe

Observations are reported of a metal-insulator transition in a 2D hole gas in asymmetrically doped strained SiGe quantum wells. The metallic phase, which appears at low temperatures in these high mobility samples, is characterised by a resistivity that decreases exponentially with decreasing temperature. This behaviour, and the duality between resistivity and conductivity on the two sides of the transition, are very similar to that recently reported for high mobility Si-MOSFETs.

cond-mat.str-el↗
arXiv1995

The Quantum Hall Effect and Inter-edge State Tunneling Within a Barrier

We have introduced a controllable nano-scale incursion into a potential barrier imposed across a two-dimensional electron gas, and report on the phenomena that we observe as the incursion develops. In the quantum Hall regime, the conductance of this system displays quantized plateaus, broad minima and oscillations. We explain these features and their evolution with electrostatic potential geometry and magnetic field as a progression of current patterns formed by tunneling between edge and localized states within the barrier.

cond-mat↗