Approximate selection rule for orbital angular momentum in atomic radiative transitions

We demonstrate that radiative transitions with Δl=−1$\mathrm{\Delta }l=-1$ are strongly dominating for all values of n and l  , except small region where l?n$l?n$.     

Martensitic phase transitions in strong magnetic fields

The model of linear chains is used to study the lattice softening of A 15-compounds in strong magnetic fields. It is shown that a strong field stabilizes the cubic phase due to the Zeeman energy of the conduction electrons. Experiments are suggested to test our theoretical findings.     

Resonant neutrino spin-flip transitions in twisting magnetic fields

We examine here in detail the dynamics of neutrino spin-flip transitions in twisting magnetic fields. The properties of various resonant conversions induced by field rotation are considered. The suppression factors for solarv _eL due to spin-flip transitions in twisting magnetic fields are calculated, and possible implications for solar neutrino experiments are discussed.     

Quantum-classical transitions in Lifshitz tails with magnetic fields

We consider Lifshitz's model of a quantum particle subject to a repulsive Poissonian random potential and address various issues related to the influence of a constant magnetic field on the leading low-energy tail of the integrated density of states. In particular, we propose the magnetic analog of a 40-year-old landmark result of Lifshitz for short-ranged single-impurity potentials U. The Lifshitz tail is shown to change its character from purely quantum, through quantum classical, to purely classical with an increasing range of U. This systematics is explained by the increasing importance of the classical fluctuations of the particle's potential energy in comparison to the quantum fluctuations associated with its kinetic energy.     

Charged scalar fields in an external magnetic field: Renormalisation and universal diamagnetism

The physical and mathematical mechanism behind diamagnetism of N (finite) spinless bosons (relativistic or non-relativistic) is well known. The mathematical signature of this diamagnetism follows from Kato's inequality while its physical way of understanding goes back to Van Leeuwen. One can guess that it might be true in the field theoretic case also. While the work on systems with a finite number of degrees of freedom suggests that the same result is true in a field theory, it does not by any means prove it. In the field theoretic context one has to develop a suitable regularisation scheme to renormalise the free energy. We show that charged scalar fields in (2+1) and (3+1) dimensions are always diamagnetic, even in the presence of interactions and at finite temperatures. This generalises earlier work on the diamagnetism of charged spinless bosons to the case of infinite degrees of freedom. We also discuss possible applications of the theory.     

Charged excitons in quantum dots: novel magnetic behavior and Auger processes

We describe theoretically multiply-charged excitons interacting with a continuum of delocalized states. Such excitons exist in relatively shallow quantum dots and have been observed in recent optical experiments on InAs self-assembled dots. The interaction of an exciton and delocalized states occurs via Auger-like processes. To describe the optical spectra, we employ the Anderson-like Hamiltonian by including the interaction between the localized exciton and delocalized states of the wetting layer. In the absence of a magnetic field, the photoluminescence line shapes exhibit interference effects. When a magnetic field is applied, the photoluminescence spectrum demonstrates anticrossings with the Landau levels of the extended states. We show that the magnetic-field behavior of charged excitons is very different to that of diamagnetic excitons in three and two-dimensional systems.     

London theory for superconducting phase transitions in external magnetic fields: application to UPt3

For multicomponent superconductors, it is known that the presence of symmetry breaking fields can lead to multiple superconducting phase transitions. Motivated by recent small angle neutron scattering experiments on the vortex state of UPt3, the London theory in the vicinity of such phase transitions is determined. It is found that the form of this London theory is in general quite different than that for conventional superconductors. This is due to the existence of a diverging correlation length associated with these phase transitions. One striking consequence is that nontrivial vortex lattices exist arbitrarily close to H(c1). Applications to UPt3, CeIn3, U(1-x)Th(x)Be(13), electron doped cuprate superconductors, Sr(2)RuO(4), and MgCNi(3) are discussed.     

Charged excitons at high magnetic fields: the effect of the surrounding electron gas

We study the photoluminescence (PL) spectrum of a two-dimensional electron system at the high magnetic field limit, where all electrons reside at the lowest Landau level (ν<2). Using a gated structure we tune the electron density from the dilute limit to a dense electron gas, and follow the changes in the emission spectrum. We find that the spectrum at the dilute limit consists of two bound triplets, whose behavior is consistent with that of the dark and bright triplets. We show that the spectrum undergoes critical changes at ν=1/3, from an isolated charged exciton-like spectrum at ν<1/3, to a spectrum that reflects the interactions with the surrounding electrons above this filling factor. This behavior is found to be robust, independent of the electron density and magnetic field. We compare our observations with other recent low temperature PL measurements of a two-dimensional electron gas at high magnetic field and find good agreement and consistency.     

Quantum transitions and magnetization of the magnetic V15 cluster in strong magnetic fields

The process of rearrangement of the magnetic structure of the low-spin cluster V₁₅ in superhigh magnetic fields is investigated. At low temperatures, this process is shown to manifest itself as three quantum jumps, each of which is a transition causing the spin of the complex to increase by two unities. The nature of these quantum jumps is discussed. The magnetization curve and the magnetic susceptibility are calculated.     

Optical absorption from indirect transitions in Ge at high magnetic fields

Our experiments show that the optical absorption from indirect transitions in Ge does not exhibit steps but consists of a series of absorption peaks at high magnetic fields. The measurements also indicate a strong Coulomb interaction in the absorption process.     

Long-lived states in solution NMR: selection rules for intramolecular dipolar relaxation in low magnetic fields

Recent work has shown that singlet states of two-spin systems in low magnetic fields can have lifetimes up to an order of magnitude longer than the usual spin-lattice relaxation time. This result may enable new applications of NMR, and in particular hyperpolarized NMR via parahydrogen-induced polarization, to the study of slow processes that take place over previously inaccessible timescales. At present it is unclear whether similar results apply to multi-spin systems, or if these long lifetimes are a peculiarity of the two-spin case. Moderately long-lived states have been observed in systems containing more than two spins, although the mechanisms that prolong their lifetimes are not well understood. Here we present formalism for the study of relaxation in multi-spin systems in low magnetic fields. This approach is used to derive a family of quantum-mechanical selection rules governing intramolecular dipolar relaxation at low field that may account for the extended lifetimes observed in multi-spin systems.     

Influence of large magnetic fields on non-radiative transitions from the A state of thiophosgene

In the present investigation we have examined fluorescence excitation spectra and fluorescence decay profiles for various vibronic bands in the X → A transitions of thiophosgene (Cl₂CS) under 10 mTorr in magnetic fields of up to 10 T. These experimental results indicated that the observed magnetic quenching (MQ) of the fluorescence can no longer be explained by the ordinary theory of the direct mechanism (DM). In order to interpret such experimental results in larger magnetic fields than 1.2 T, we have applied a new theory that was developed by solving the equation of motion for the density matrix. According to this theory, MQ due to DM is saturated in sufficiently large fields. This theoretical prediction is in agreement with the experimental results for the 4⁴ ₁, 4¹ ₀, 3¹ ₀4¹ ₀, 4³ ₀, and 1¹ ₀4¹ ₀ bands. Thus, we may conclude that the saturation of MQ due to DM is a new phenomenon characteristic of MQ in magnetic fields larger than 1.2 T. Moreover, we have examined the pressure dependence of MQ of the fluorescence in several vibronic bands. These experimental results show that MQ of the fluorescence under 10 mTorr is due mainly to the acceleration of intramolecular radiationless processes induced by magnetic fields.     

Induced magnetic phase transitions in rare-earth intermetallic compounds RMn2Ge2 in ultrastrong magnetic fields

The differential magnetic susceptibility of intermetallic compounds RMn₂Ge₂ (R=Gd, Tb, Dy, Ho, Y) with a layered tetragonal structure is measured in pulsed magnetic fields up to 130 T. It is found that all these compounds undergo a first-order magnetic phase transition in strong magnetic fields. The nature of this transition is discussed, and it is found that a change in the magnetic state of the manganese sublattice is responsible for the transition.     

Numerical study on Anderson transitions in three-dimensional disordered systems in random magnetic fields

The Anderson transitions in a random magnetic field in three dimensions are investigated numerically. The critical behavior near the transition point is analyzed in detail by means of the transfer matrix method with high accuracy for systems both with and without an additional random scalar potential. We find the critical exponent ν for the localization length to be 1.45 ± 0.09 with a strong random scalar potential. Without it, the exponent is smaller but increases with the system sizes and extrapolates to the above value within the error bars. These results support the conventional classification of universality classes due to symmetry. Fractal dimensionality of the wave function at the critical point is also estimated by the equation-of-motion method.     

Propensity rule for novel selective double photoexcitation of helium atoms in strong static electric fields

We studied the double photoexcitation spectra of helium in a strong dc electric field and compared the results with the recent experimental data of Harries et al. [Phys. Rev. Lett. 90, 133002 (2003)]]. We derived the propensity rules based on the crossing or noncrossing of energies in the Stark map to predict the selective subset of doubly excited states that are preferentially populated in such experiments. It is shown that the propensity rule is a consequence of the ubiquitous correlation properties of doubly excited states in general.     

Induction of ferroelectric transitions by the noncollinear exchange-modulated magnetic structure and magnetic fields in terbium manganate

The magnetic transition from the exchange-modulated collinear to the noncollinear state in terbium manganate, accompanied by the appearance of electric polarization, is explained within the Landau theory of phase transitions. The experimentally observed reorientation or disappearance of electric polarization in magnetic fields along the Y and Z axes are explained by the spin-flop transitions of manganese spins from the incommensurate noncollinear to the commensurate magnetic phase.