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P. John

Publications and source records attributed to P. John.

11 recordsLinked to original sources
arXiv2024

AlN Nanowire Based Vertically Integrated Piezoelectric Nanogenerators

In this study, detailed analysis of the direct piezo-response of AlN nanowire-based vertically integrated nanogenerators (VINGs) is undertaken as a function of mechanical excitation frequency. We show that the piezo-charge, piezo-voltage, and impedance measured at the same position of the devices can be directly correlated through an equivalent circuit model, in the whole frequency range of investigation. Our presented results are utilized to determine the performance figures of merit (FoM) of nanowire-based VINGs, namely the piezoelectric voltage constant (g) for sensing, and the product d g for energy harvesting, where d is the piezoelectric charge constant. By comparison of these metrics with those of freestanding single crystal GaN and quartz substrates, as well as sputtered AlN thin films, we suggest that the nanowires can outperform their rigid counterparts in terms of mechanical sensing and energy generation. This work provides experimental guidelines for understanding the direct piezo-characteristics of VINGs and facilitates a quantitative comparison between nanostructured piezoelectric devices fabricated using different materials or architectures.

physics.app-ph↗
arXiv2023

Particle Identification at VAMOS++ with Machine Learning Techniques

Multi-nucleon transfer reaction between 136Xe beam and 198Pt target was performed using the VAMOS++ spectrometer at GANIL to study the structure of n-rich nuclei around N=126. Unambiguous charge state identification was obtained by combining two supervised machine learning methods, deep neural network (DNN) and positional correction using a gradient-boosting decision tree (GBDT). The new method reduced the complexity of the kinetic energy calibration and outperformed the conventional method, improving the charge state resolution by 8%

physics.ins-det↗
arXiv2004

The role of C2 in nanocrystalline diamond growth

This paper presents findings from a study of nanocrystalline diamond (NCD) growth in a microwave plasma chemical vapour deposition (CVD) reactor. NCD films were grown using Ar/H2/CH4 and He/H2/CH4 gas compositions. The resulting films were characterised using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. Analysis revealed an estimated grain size of the order of 50 nm, growth rates in the range 0.01 to 0.3 um/h and sp3 and sp2 bonded carbon content consistent with that expected for NCD. The C2 Swan band was probed using cavity ring-down spectroscopy (CRDS) to measure the absolute C2 (a) number density in the plasma during diamond film growth. The number density in the Ar/H2/CH4 plasmas was in the range 2 to 4 x 10^12 cm-3, but found to be present in quantities too low to measure in the He/H2/CH4 plasmas. Optical emission spectrometry (OES) was employed to determine the relative densities of the C2 excited state (d) in the plasma. The fact that similar NCD material was grown whether using Ar or He as the carrier gas suggests that C2 does not play a major role in the growth of nanocrystalline diamond.

cond-mat.mtrl-sci↗
arXiv2002

Do Stops Slow Down Electroweak Bubble Walls?

We compute the wall velocity in the MSSM. We therefore generalize the SM equations of motion for bubble walls moving through a hot plasma at the electroweak phase transition and calculate the friction terms which describe the viscosity of the plasma. We give the general expressions and apply them to a simple model where stops, tops and W bosons contribute to the friction. In a wide range of parameters including those which fulfil the requirements of baryogenesis we find a wall velocity of order v = 0.05-0.1 much below the SM value.

hep-ph↗
arXiv2001

One-loop Higgs mass finiteness in supersymmetric Kaluza-Klein theories

We analyze the one-loop ultraviolet sensitivity of the Higgs mass in a five-dimensional supersymmetric theory compactified on the orbifold S^1/Z_2, with superpotential localized on a fixed-point brane. Four-dimensional supersymmetry is broken by Scherk-Schwarz boundary conditions. Kaluza-Klein interactions are regularized by means of a brane Gaussian distribution along the extra dimension with length l_s\simeqΛ^{-1}_s, where Λ_s is the cutoff of the five-dimensional theory. The coupling of the n-mode, with mass M^{(n)}, acquires the n-dependent factor exp{-(M^{(n)}/Λ_s)^2/2}, which makes it to decouple for M^{(n)}\gg Λ_s. The sensitivity of the Higgs mass on Λ_s is strongly suppressed and quadratic divergences cancel by supersymmetry. The one-loop correction to the Higgs mass is finite and equals, for large values of Λ_s, the value obtained by the so-called KK-regularization.

hep-ph↗
arXiv2001

Bubble Walls, CP Violation and Electroweak Baryogenesis in the MSSM

We discuss the generation of the baryon asymmetry by a strong first order electroweak phase transition in the early universe, particularly in the context of the MSSM. This requires a thorough numerical treatment of the bubble wall profile in the case of two Higgs fields. CP violating complex particle masses varying with the Higgs field in the wall are essential. Since in the MSSM there is no indication of spontaneous CP violation around the critical temperature (contrary to the NMSSM) we have to rely on standard explicit CP violation. Using the WKB approximation for particles in the plasma we are led to Boltzmann transport equations for the difference of left-handed particles and their CP conjugates. This asymmetry is finally transformed into a baryon asymmetry by out of equilibrium sphaleron transitions in the symmetric phase. We solve the transport equations and find a baryon asymmetry depending mostly on the CP violating phases and the wall velocity.

hep-ph↗
arXiv2000

Wall Velocity in the MSSM

We compute the wall velocity in the MSSM with W, tops and stops contributing to the friction. In a wide range of parameters including those which fulfil the requirements of baryogenesis we find a wall velocity of order v_w=O(0.01) much below the SM value.

hep-ph↗
arXiv1999

CP Violating Bubble Wall Profiles

We solve the equations of motion for a CP violating phase between the two Higgs doublets at the bubble wall of the MSSM electroweak phase transition. Contrary to earlier suggestions, we do not find indications of spontaneous ``transitional'' CP violation in the MSSM. On the other hand, in case there is explicit CP violation in the stop and chargino/neutralino sectors, the relative phase between the Higgses does become space dependent, but only mildly even in the maximal case. We also demonstrate that spontaneous CP violation within the bubble wall could occur, e.g., if the Higgs sector of the MSSM were supplemented by a singlet. Finally we point out some implications for baryogenesis computations.

hep-ph↗
arXiv1996

The 2-loop MSSM finite temperature effective potential with stop condensation

We calculate the finite temperature 2-loop effective potential in the MSSM with stop condensation, using a 3-dimensional effective theory. We find that in a part of the parameter space, a two-stage electroweak phase transition appears possible. The first stage would be the formation of a stop condensate, and the second stage is the transition to the standard electroweak minimum. The two-stage transition could significantly relax the baryon erasure bounds, but the parameter space allowing it (m_H \lsim 100 GeV, m_tR \sim 155-160 GeV) is not very large. We estimate the reliability of our results using renormalization scale and gauge dependence. Finally we discuss some real-time aspects relevant for the viability of the two-stage scenario.

hep-ph↗
arXiv1996

Algebraic characterization of gauge anomalies on a nontrivial bundle

We discuss the algebraic way of solving the descent equations corresponding to the BRST consistency condition for the gauge anomalies and the Chern--Simons terms on a nontrivial bundle. The method of decomposing the exterior derivative as a BRST commutator is extended to the present case.

hep-th↗