113 research outputs found
Synthetic spin-orbit coupling mediated by a bosonic environment
We study a mobile quantum impurity, possessing internal rotational degrees of
freedom, confined to a ring in the presence of a many-particle bosonic bath. By
considering the recently introduced rotating polaron problem, we define the
Hamiltonian and examine the energy spectrum. The weak-coupling regime is
studied by means of a variational ansatz in the truncated Fock space. The
corresponding spectrum indicates that there emerges a coupling between the
internal and orbital angular momenta of the impurity as a consequence of the
phonon exchange. We interpret the coupling as a phonon-mediated spin-orbit
coupling and quantify it by using a correlation function between the internal
and orbital angular momentum operators. The strong-coupling regime is
investigated within the Pekar approach and it is shown that the correlation
function of the ground state shows a kink at a critical coupling, that is
explained by a sharp transition from the non-interacting state to the states
that exhibit strong interaction with the surroundings. The results might find
applications in such fields as spintronics or topological insulators, where
spin-orbit coupling is of crucial importance.Comment: 8 pages, 5 figure
Impurity with a resonance in the vicinity of the Fermi energy
We study an impurity with a resonance level whose position coincides with the Fermi energy of the surrounding Fermi gas. An impurity causes a rapid variation of the scattering phase shift for fermions at the Fermi surface, introducing a new characteristic length scale into the problem. We investigate manifestations of this length scale in the self-energy of the impurity and in the density of the bath. Our calculations reveal a model-independent deformation of the density of the Fermi gas, which is determined by the width of the resonance. To provide a broader picture, we investigate time evolution of the density in quench dynamics, and study the behavior of the system at finite temperatures. Finally, we briefly discuss implications of our findings for the Fermi-polaron problem
Peculiarities of dynamics of Dirac fermions associated with zero-mass lines
Zero-mass lines result in appearance of linear dispersion modes for Dirac
fermions. These modes play an important role in various physical systems.
However, a Dirac fermion may not precisely follow a single zero-mass line, due
to either tunneling between different lines or centrifugal forces. Being
shifted from a zero-mass line the Dirac fermion acquires mass which can
substantially influence its expected "massless" behavior. In the paper we
calculate the energy gap caused by the tunneling between two zero-mass lines
and show that its opening leads to the delocalization of linear dispersion
modes. The adiabatic bending of a zero-mass line gives rise to geometric
phases. These are the Berry phase, locally associated with a curvature, and a
new phase resulting from the mass square asymmetry in the vicinity of a
zero-mass line.Comment: 6 pages, 4 figures. In the second version some references were added
and minor changes were made in the introductio
Particle transport in JET and TCV H-mode plasmas
Understanding particle transport physics is of great importance for magnetically confined plasma devices and for the development of thermonuclear fusion power for energy production. From the beginnings of fusion research, more than half a century ago, the problem of heat transport in tokamaks attracted the attention of researchers, but the particle transport phenomena were largely neglected until fairly recently. As tokamak physics advanced to its present level, the physics community realized that there are many hurdles to the development of fusion power beyond the energy confinement. Particle transport is one of the outstanding issues. The aim of this thesis work is to study the anomalous (turbulence driven) particle transport in tokamaks on the basis of experiments on two different devices: JET (Joint European Torus) and TCV (Tokamak Ă Configuration Variable). In particular the physics of particle inward convection (pinch), which causes formation of peaked density profiles, is addressed in this work. Density profile peaking has a direct, favorable effect on fusion power in a reactor, we therefore also propose an extrapolation to the international experimental reactor ITER, which is currently under construction. To complete the thesis research, a comprehensive experimental database was created on the basis of data collected from on JET and TCV during the duration of the thesis. Improvements of the density profile measurements techniques and careful analysis of the experimental data allowed us to derive the dependencies of density profile shape on the relevant plasma parameters. These improved techniques also allowed us to dispel any doubts that had been voiced about previous results. The major conclusions from previous work on JET and other tokamaks were generally confirmed, with some minor supplements. The main novelty of the thesis resides in systematic tests of the predictions of linear gyrokinetic simulations of the ITG (Ion Temperature Gradient) mode against the experimental observations. The simulations were performed with the GS2 code. The parameter dependencies of plasma density gradient, as observed on JET, are a good agreement with those from the simulations over a wide range. The simulations done for the TCV case, which are in a different physics parameter domain, are also in partial agreement with the experiment. Complete agreement is not out of the question, but will remain a goal for the future, when measurements of one of the most important parameters will be available. The good agreement between the experiment and the simulations suggests that the ITG instability may be responsible for the majority of the anomalous inward particle convection observed in tokamaks. Both the simulations and the empirical data extrapolations predicting a peaked density profile in ITER plasma conditions, instead of a flat one, as was assumed during the concept design period
Density profile peaking in JET H-mode plasmas: experiments versus linear gyrokinetic predictions
As an independent complement to previous studies (Weisen et al 2005 Nucl. Fusion 45 L1-4, Weisen et al 2006 Plasma Phys. Control. Fusion 48 A457-66, Angioni et al 2007 Nucl. Fusion 47 1326-35), density peaking in the JET tokamak was investigated on the dataset, comprising virtually all H-mode experiments performed in 2006-2007. Unlike previous studies, this work focuses on low collisionality data as most representative of reactor conditions. The study confirms that collisionality is the most important parameter governing density peaking in H-mode, followed by the NBI particle flux and/or the T-i/T-e temperature ratio. For the first time in JET a modest, albeit significant dependence of peaking on internal inductance, or magnetic shear is seen. The experimental behaviour is compared with an extensive database of linear gyrokinetic calculations using the GS2 code. The predictions from GS2 simulations based on the highest linear growth rate mode are in good agreement with experimental observations. They are also corroborated by initial results from the non-linear code GYRO
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