Carrier dynamics in ion-implanted GaAs studied by simulation and observation of terahertz emission

We have studied terahertz (THz) emission from arsenic-ion implanted GaAs both experimentally and using a three-dimensional carrier dynamics simulation. A uniform density of vacancies was formed over the optical absorption depth of bulk GaAs samples by performing multi-energy implantations of arsenic ions (1 and 2.4MeV) and subsequent thermal annealing. In a series of THz emission experiments the frequency of peak THz power was found to increase significantly from 1.4 to 2.2THz when the ion implantation dose was increased from 10^13 to 10^16 cm-3. We used a semi-classical Monte-Carlo simulation of ultra-fast carrier dynamics to reproduce and explain these results. The effect of the ion-induced damage was included in the simulation by considering carrier scattering at neutral and charged impurities, as well as carrier trapping at defect sites. Higher vacancy concentrations and shorter carrier trapping times both contributed to shorter simulated THz pulses, the latter being more important over experimentally realistic parameter ranges.Comment: 6 pages, 7 figure

Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the nu F_nu Spectrum of Gamma Ray Bursts

We present results of correlation studies, examining the association between the peak of the nu F_nu spectrum of gamma ray bursts, E_p, with the burst's energy fluence and photon peak flux. We discuss methods to account for data truncation in E_p and fluence or flux when performing the correlation analyses. However, because bursts near the detector threshold are not usually able to provide reliable spectral parameters, we focus on results for the brightest bursts in which we can better understand the selection effects relevant to E_p and burst strength. We find that there is a strong correlation between total fluence and E_p. We discuss these results in terms of both cosmological and intrinsic effects. In particular, we show that for realistic distributions of the burst parameters, cosmological expansion alone cannot account for the correlation between E_p and total fluence; the observed correlation is likely a result of an intrinsic relation between the burst rest-frame peak energy and the total radiated energy. We investigate this latter scenario in the context of synchrotron radiation from external and internal shock models of GRBs. We find that the internal shock model is consistent with our interpretation of the correlation, while the external shock model cannot easily explain this intrinsic relation between peak energy and burst radiated energy.Comment: 23 pages, including 8 postscript figures. Submitted to Ap

The appropriateness of psychotropic medicines: an interview study of service users attending a substance misuse service in England

Background Mental health problems are common in people with substance misuse problems. However, there is a paucity of evidence regarding prescribing of psychotropic medications for people with comorbid mental health and substance misuse problems. Objective To explore the views of service users attending an addiction service on the appropriateness of psychotropic medications prescribed for their co-existing mental health problems. Setting A specialist addiction service in the North of England. Method A phenomenological approach was adopted. Semi-structured interviews were conducted with twelve service users. Data were analysed using thematic framework analysis. Main outcome measure Service users’ views concerning the appropriateness of their prescribed psychotropic medications. Results The following themes captured service users’ views on the appropriateness of their medications: benefits from medicines, entitlement to medicines, and assessment and review. Service users mostly described benefits from their medications (including those prescribed outside guideline recommendations) and there was also an awareness of the adverse effects they experienced from them. It appears that people with substance misuse problems have a particularly strong sense of their own needs and seek to influence prescribing decisions. Service users further described varied practices regarding assessment and review of their medications with evidence of regular reviews while others identified suboptimal or inadequate practices. Conclusion Most service users described improved functioning as a result of their prescribed psychotropic medications. Prescriptions that are inappropriate in terms of their usual indications may well be justified if they assist in stabilising service users and moving them on to recovery

Polarisation-sensitive terahertz detection by multicontact photoconductive receivers

We have developed a terahertz radiation detector that measures both the amplitude and polarization of the electric field as a function of time. The device is a three-contact photoconductive receiver designed so that two orthogonal electric-field components of an arbitrary polarized electromagnetic wave may be detected simultaneously. The detector was fabricated on Fe+ ion-implanted InP. Polarization-sensitive detection is demonstrated with an extinction ratio better than 100:1. This type of device will have immediate application in studies of birefringent and optically active materials in the far-infrared region of the spectrum.Comment: 3 pages, 3 figure

Tracking a hysteretic and disorder-broadened phase transition via the electromagnon response in improper ferroelectrics

We demonstrate that electromagnons can be used to directly probe the nature of a phase transition between magnetically ordered phases in an improper ferroelectric. The antiferromagnetic/paraelectric to antiferromagnetic/ferroelectric phase transition in Cu1-xZnxO (x=0, 0.05) alloys was tracked via the electromagnon response using terahertz time-domain spectroscopy, on heating and cooling through the phase transition. The transition was found to exhibit thermal hysteresis, confirming its first-order nature, and to broaden under the influence of spin-disorder upon Zn substitution. The energy of the electromagnon increases upon alloying, as a result of the non-magnetic ions modifying the magnetic interactions that give rise to the multiferroic phase and electromagnons. We describe our findings in the context of recent theoretical work that examined improper ferroelectricity and electromagnons in CuO from phenomenological and first-principles approaches

Quantum computation over continuous variables

This paper provides necessary and sufficient conditions for constructing a universal quantum computer over continuous variables. As an example, it is shown how a universal quantum computer for the amplitudes of the electromagnetic field might be constructed using simple linear devices such as beam-splitters and phase shifters, together with squeezers and nonlinear devices such as Kerr-effect fibers and atoms in optical cavities. Such a device could in principle perform `quantum floating point' computations. Problems of noise, finite precision, and error correction are discussed.Comment: 9 pages, Te

Narrow heavy-hole cyclotron resonances split by the cubic Rashba spin-orbit interaction in strained germanium quantum wells

The spin-orbit interaction was found to split the cyclotron resonance of heavy holes confined in high-mobility, compressively strained germanium quantum wells. The interference between coherent spin-split cyclotron resonances was tracked on picosecond time scales using terahertz time-domain spectroscopy. Analysis in the time domain, or using a time-frequency decomposition based on the Gabor-Morlet wavelet, was necessary when the difference between cyclotron frequencies was comparable to the linewidth. The cubic Rashba spin-orbit coefficient β was determined via two methods: (i) the magnetic-field dependence of the cyclotron frequencies, and (ii) the spin-resolved subband densities. An enhanced β and spin polarization was created by tailoring the strain to enhance the spin-orbit interaction. The amplitude modulation of the narrow, interfering cyclotron resonances is a signature of spin coherences persisting for more than 10 ps

Prescribers' views and experiences of assessing the appropriateness of prescribed medications in a specialist addiction service

Background: Mental and physical health problems are common in people with substance misuse problems and medications are often required in their management. Given the extent of prescribing for service users who attend specialist addiction services, it is important to consider how prescribers in this setting assess the appropriateness of service users’ prescribed medications. Objective: To explore prescribers’ views and experiences of assessing the appropriateness of medications prescribed for service users coming in for treatment as well as the differences between prescribers. Setting: A specialist addiction service in the North of England. Method: A phenomenological approach was adopted. Semi-structured interviews were conducted with four nurse prescribers and eight doctors. Data were analysed using thematic framework analysis. Main outcome measure: Prescribers’ views and experiences of assessing the appropriateness of prescribed medications. Results: Assessment of the appropriateness of prescribed medications involved reviewing medications, assessing risk, history-taking, involvement of service users, and comparing guideline adherence and ‘successful’ prescribing. Doctors and nurse prescribers assessed the appropriateness of medications they considered to be within their competency. Doctors provided support to nurse prescribers and general practitioners (GPs) when dealing with issues around prescribing. Conclusion: Assessment of the appropriateness of prescribed medications is complex. The recent reduction in medical expertise in specialist addiction services may negatively impact on the clinical management of service users. It appears that there is a need for further training of nurse prescribers and GPs so they can provide optimal care to service users