Landau Level System Research Articles

Instability Due to Electron Captures of Nuclei 56Ni, 56Co, 56Fe, 56Mn, 56Cr, and 56V in Strongly Magnetized Super-Chandrasekhar White Dwarfs

Based on the theory of the relativistic mean-field effective interactions, we explore the effect of superstrong magnetic fields (SMFs) on nuclear single-particle level structure, nuclear blinding energy, and the electron Fermi energy in strongly magnetized super-Chandrasekhar white dwarfs (SMSWDs). We also discuss the electron capture (EC) instability from the threshold mass density (TMD) and threshold pressure (TP) for some typical iron group nuclei in SMSWD due to SMFs. At relatively low densities (e.g., ρ 7 = 57, 97, 197), the EC rates increase and they then decrease by more than four orders of magnitude. However, at relatively higher density (e.g., ρ 7 = 4197), the rates increase by about two orders of magnitude when 2 × 103 < B 12 < 5 × 104, and then the SMFs have a minor effect on EC rates. The TMD (TP) can increase by about 23% ∼ 29%, due to an increase in the electron–ion interaction and nuclear binding energy in SMFs. As SMFs further increase, the TMD (TP) increase almost linearly for one Landau levels system. For a two and three Landau level system, the SMFs have a slight influence on the TMD (TP) (e.g., at B 12 = 103.5), and then TMD (TP) increases by about 13% ∼ 14%, and finally decreases by about 27.85% (27.34%). The shift of the onset of EC can make the maximum mass of the SMSWD increase by more than 2.28% for a one Landau level system. This influence from TMD (TP) can be larger than that of general relativity for SMSWD, which contains some iron group nuclei.

Ability to emit stimulated terahertz radiation between Landau levels in GaAs/Al GaAs quantum well

In this paper we show that a population inversion can be achieved in the system of Landau levels in quantum well structures by a considerable difference between intersubband and intrasubband electron-electron scattering rate. This mechanism allows the generation of emission terahertz stimulated radiation. The frequency of the radiation may continuously be tuned in a wide range of terahertz frequencies by the variation of the magnetic field strength.

The kinectics of energy relaxation in quantum wells in a quantizing magnetic field

The kinetics of the intraband relaxation of the electron energy in the system of Landau levels lying below the the optical phonon energy is studied. Unusual behaviour of the relaxation of electronic subsystem is found. Even though its main channel is the emission of optical phonons, the total relaxation time is several orders of magnitude higher than the characteristic scattering times on optical phonons.

Role of guiding center in Landau level system and mechanical and pseudo orbital angular momenta

There is an interesting but not-so-popular quantity called pseudo orbital angular momentum (OAM) in the Landau-level system, besides the well-known canonical and mechanical OAMs. The pseudo OAM can be regarded as a gauge-invariant extension of the canonical OAM, which is formally gauge invariant and reduces to the canonical OAM in a certain gauge. Since both of the pseudo OAM and the mechanical OAM are gauge invariant, it is impossible to judge which of those is superior to the other solely from the gauge principle. However, these two OAMs have totally different physical meanings. The mechanical OAM shows manifest observability and clear correspondence with the classical OAM of the cyclotron motion. On the other hand, we demonstrate that the standard canonical OAM as well as the pseudo OAM in the Landau problem is the concept which crucially depend on the choice of the origin of the coordinate system. We try to reveal the relation between the pseudo OAM and the mechanical OAM as well as their observability by paying special attention to the role of guiding-center operator in the Landau problem.

Theoretical Models of Highly Magnetic White Dwarf Stars with Non-Polytropic Equation of State

Super-massive white dwarf (WD) stars in the mass range 2.4 - 2.8 solar masses are believed to be the progenitors of “super-luminous” Type Ia supernovae according to a hypothesis proposed by some researchers. They theorize such a higher mass of the WD due to the presence of a very strong magnetic field inside it. We revisit their first work on magnetic WDs (MWDs) and present our theoretical results that are very different from theirs. The main reason for this difference is in the use of the equation of state (EoS) to make stellar models of MWDs. An electron gas in a magnetic field is Landau quantized and hence, the resulting EoS becomes non-polytropic. By constructing models of MWDs using such an EoS, we highlight that a strong magnetic field inside a WD would make the star super-massive. We have found that our stellar models do indeed fall in the mass range given above. Moreover, we are also able to address an observational finding that the mean mass of MWDs are almost double that of non-magnetic WDs. Magnetic field changes the momentum-space of the electrons which in turn changes their density of states (DOS), and that in turn changes the EoS of matter inside the star. By correlating the magnetic DOS with the non-polytropic EoS, we were also able to find a physical reason behind our theoretical result of super-massive WDs with strong magnetic fields. In order to construct these models, we have considered different equations of state with at most three Landau levels occupied and have plotted our results as mass-radius relations for a particular chosen value of maximum Fermi energy. Our results also show that a multiple Landau-level system of electrons leads to such an EoS that gives multiple branches in the mass-radius relations, and that the super-massive MWDs are obtained when the Landau-level occupancy is limited to just one level. Finally, our theoretical results can be explained solely on the basis of quantum and statistical mechanics that warrant no assumptions regarding stars.

Recombination dynamics of Landau levels in an InGaAs/InP quantum well

The dynamics of differently spin-polarized carriers photoexcited in a system of Landau levels is investigated in an InGaAs/InP quantum well. Shake-up emission from Landau levels above the Fermi level is observed, and it is shown to significantly affect the recombination dynamics of Landau levels. The shake-up effect is found to occur due to the inter-Landau-level scattering, which manifests itself in a rapid decay of the emission from Landau levels. In addition, the inter-Landau-level scattering is shown to determine the time delay of photoluminescence response due to the relaxation of photoexcited electrons among Landau levels. The sharp minimum of the recombination time is observed between the quantum Hall phases with the filling factors $\ensuremath{\nu}=1$ and 2, and it is attributed to the formation of the metallic intermediate phase, which causes increasing overlap between the electron and hole wave functions.

Kinetics of intrasubband electron energy relaxation in quantum wells in a quantizing magnetic field

Herein we report the intrasubband relaxation of electron energy in the Landau level system of quantum well as a function of magnetic field strength. The relative role of scattering processes contributing to relaxation kinetics, the electron-electron scattering which redistributes the electrons between Landau levels and the optical phonon emission by the electrons reaching the Landau levels lying near and above the optical phonon energy is revealed. It was shown the most important factor determining the kinetics of energy relaxation is the electron-electron scattering processes, delivering electrons to Landau levels close to optical phonon energy. The flux of these electrons depends on the number of the Landau levels lying below the energy of the optical phonon and increases substantially with a decrease in this number. On the contrary, the electron-phonon scattering rate dependence of the energy relaxation time was found to be much weaker.

Paired states in half-filled Landau levels

We discuss monolayer and bilayer quantum Hall systems in which each layer is a half-filled Landau level (LL) system. In the mean field approximation of the Son's formalism there is a common pairing structure that underlines the possibilities for paired ground states in both systems. We argue that the particle-hole (PH) Pfaffian state in the (particle-hole symmetric) half-filled LL of a monolayer, and analogous state in the PH symmetric bilayer (in which each layer is half-filled LL) can be considered as {\em critical states} i.e. states that cannot describe a phase under PH symmetry. We point out that the inclusion of a PH symmetry breaking (like LL mixing) may stabilize the PH Pfaffian in a monolayer. In the bilayer case, in numerical experiments on a sphere, by choosing the PH symmetric shift, we can stabilize the interlayer correlated (111) excitonic state or critical state, for any distance between the layers, but in general, with no bias, the evolution of the bilayer includes other phases.

Giant frequency tunability enabled by external magnetic and a gate electric fields in graphene devices.

Graphene possesses a unique Landau level system that is non-equidistantly spaced in energy, as thus a large amount of optical transitions may become possible. Here, by utilizing this unique feature, we propose a novel dual field method which combines both external magnetic field and gate electric field together to control the optical response of the graphene-based devices. The key principle of this method is to selectively allow different optical transitions in graphene among Landau levels via an electric gate tuning of the Fermi level. By applying this method to a graphene based amplitude modulator and through an implementation based on transfer matrix method, we numerically demonstrated the well characteristics of switchable modulation on four individual channels, a huge modulation depth up to 80 dB and an extremely low required energy of tuning Fermi level down to 10 meV. Such excellent frequency tunability and gate controlling ability of this dual field method may open up the potential for applications in active optoelectronics, spin optics, ultrafast optics and etc.

Energy relaxation mechanism in Landau level system of quantum wells

Kinetics of intrasubband energy relaxation of electrons in the system of Landau levels lying below the optical phonon energy is studied. Extraordinary behavior of the relaxation of electronic subsystem excitation energy is detected. Despite the fact that its main channel is optical phonon emission, the total relaxation time exceeds the characteristic times of scattering on optical phonons by several orders of magnitude.

Mechanism of energy relaxation in the system of Landau levels in quantum wells

The kinetics of intersubband relaxation of electron energy has been studied in the system of Landau levels lying below the optical phonon energy. The relaxation character in the considered system is revealed to differ qualitatively from that in the two-dimensional continuous subband of the quantum well. In particular, the mechanisms of electron subsystem thermalization and energy relaxation in the system of Landau levels are qualitatively different, and the electron subsystem relaxation time exceeds the thermalization time by several orders of magnitude.

Intersubband population inversion in landau level system in resonant tunneling quantum well structures with asymmetric double quantum well period

A resonant tunneling periodic quantum well structure with asymmetric double quantum well period is proposed to achieve a population inversion and a frequency-tunable terahertz emission on intersubband transitions between Landau levels in a tilted magnetic field. The spatial redistribution of the subband wavefunctions caused by an asymmetric period design made it possible to achieve both a considerable increase in the population inversion and a high value of the dipole matrix element of the inter-Landau level radiative transition.

The mass limit of white dwarfs with strong magnetic fields in general relativity

Recently, U. Das and B. Mukhopadhyay proposed that the Chandrasekhar limit of a white dwarf could reach a new high level (2.58M⊙) if a superstrong magnetic field were considered (Das U and Mukhopadhyay B 2013 Phys. Rev. Lett. 110 071102), where the structure of the strongly magnetized white dwarf (SMWD) is calculated in the framework of Newtonian theory (NT). As the SMWD has a far smaller size, in contrast with the usual expectation, we found that there is an obvious general relativistic effect (GRE) in the SMWD. For example, for the SMWD with a one Landau level system, the super-Chandrasekhar mass limit in general relativity (GR) is approximately 16.5% lower than that in NT. More interestingly, the maximal mass of the white dwarf will be first increased when the magnetic field strength keeps on increasing and reaches the maximal value M = 2.48M⊙ with BD = 391.5. Then if we further increase the magnetic fields, surprisingly, the maximal mass of the white dwarf will decrease when one takes the GRE into account.

Carrier kinetics and population inversion in Landau level system in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau levels population for various values of pumping intensity (tunneling time), magnetic field and the structure doping were carried out. The effect of various scattering mechanisms, as two-electron (electron–electron scattering) as single-electron (acoustic phonon and interface roughness scattering) ones on level population was studied. The population inversion between the zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength thus providing the possibility of wide range tunable stimulated terahertz emission.

WITHDRAWN: Intersubband terahertz transitions in Landau level system of cascade GaAs/AlGaAs quantum well structures in strong tilted magnetic field

WITHDRAWN: Intersubband terahertz transitions in Landau level system of cascade GaAs/AlGaAs quantum well structures in strong tilted magnetic field

Intersubband terahertz transitions in Landau level system of cascade GaAs/AlGaAs quantum well structures in strong tilted magnetic field

The tunable terahertz intersubband Landau level transitions in resonant tunneling cascade quantum well structures are considered. The way of lifting the selection rule forbidding the inter-Landau level terahertz transitions of interest by applying a magnetic field tilted with respect to the structure layers is proposed. The importance of asymmetric structure design to achieve considerable values of transition dipole matrix elements is demonstrated.

ECHO EFFECTS ON RELATIVISTIC LANDAU LEVELS IN GRAPHENE AND BIGRAPHENE AS A MANIFESTATION OF THE QUANTUM MEMORY

We consider the echo effects, which can take place in graphene and bigraphene (bilayer graphene), when the system of relativistic Landau levels in a quantizing magnetic field appears. Graphene (bigraphene) is examined theoretically in the long-wave approximation near the Dirac points. We propose to use the echo effects for realization of quantum memory for optical states in the far-infrared region.

Intersubband population inversion and stimulated terahertz transitions between Landau levels in resonant tunneling multiple quantum well structures

It is shown that a population inversion can be achieved in the system of Landau levels in cascade quantum well structures in a magnetic field under a condition of sequential resonant tunneling. This mechanism allows a wide range tuning of the emitted terahertz frequency by the magnetic field strength variation.

Intersubband population inversion and induced transitions between the Landau levels in resonance-tunneling multiple quantum-well structures

A new mechanism of creating the population inversion in the system of Landau levels in resonance-tunneling multiple quantum-well structures has been proposed. It has been shown that when the distance between the lower subbands is smaller than the energy of an optical phonon (i.e., when the scattering by optical phonons is suppressed), the population of the ground (zeroth) Landau level of the upper subband can be substantially higher than the population of the first Landau level of the lowest subband, which opens the opportunity of obtaining the tunable-frequency generation of induced terahertz electromagnetic radiation at such a transition.

Noncommutative superspace, supermatrix and lowest Landau level

By using graded (super-)Lie algebras, we can construct noncommutative superspace on curved homogeneous manifolds. In this paper, we take a flat limit to obtain flat noncommutative superspace. We particularly consider d=2 and d=4 superspaces based on the graded Lie algebras osp(1|2), su(2|1) and psu(2|2). Jacobi identities of supersymmetry algebras and associativities of star products are automatically satisfied. Covariant derivatives which commute with supersymmetry generators are obtained and chiral constraints can be imposed. We also discuss that these noncommutative superspaces can be understood as constrained systems analogous to the lowest Landau level system.