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Mladen Georgiev

Publications and source records attributed to Mladen Georgiev.

At least 37 records · Page 2Linked to original sources
arXiv2008

In the quest for a global (super)symmetry breaking mechanism: an extension of the vibronic mixing process to superpartners

We propose a global symmetry-breaking mechanism based on an extension of the vibronic (cooperative Jahn-Teller) effects (comprising electron-phonon superpartners) to generalize them into fermion-boson superpartners. Examples of superpairs are constructed by combining phonons, photons, pi-mesons, etc. with electrons, holes, nucleons, etc. in condensed and nuclear matter. The obtained symmetry-breaking mechanism operating amidst the superpartner pairs may be just what is needed to justify the status of supersymmetry as a broken-symmetry phenomenon. Following the extended vibronic mixing of fermion states by the appropriate heavy boson superpartner, the latter propagates along a helical path away from the mixing site.

physics.class-ph↗
arXiv2008

Note on the oblate and prolate deformations in nuclear matter from the viewpoint of the quantum-mechanical off-center effect

We consider the possibility that a quantum-mechanical off-center effect may be behind the deformed oblate and prolate shapes of nuclei in nuclear physics. In solid state physics, finite off-center displacements result from the mixing of electronic states through their coupling to vibrational (phonon) modes of appropriate symmetries. This is an example of fermion-boson interaction which may materialize in nuclear physics as well in the form of a coupling of nucleons to the pi-meson field. We carry out calculations to substantiate the proposal.

nucl-th↗
arXiv2008

Letter for the pressure effects on the electrostatic polarizability and dispersive binding of off-center polarons in ionic and molecular systems

The question is raised about the pressure effects on the polarizability and dispersive binding in ionic and molecular systems. As an example we take the effect of hydrostatic pressure on the energy gap of phonon-coupled two-level systems. Assuming a positive pressure coefficient for the gap, we predict a gradual transition from small vibronic polarons to large polarons as the pressure is increased at constant coupling energy of a two-level system. Concomitantly, the universal dispersive binding degrades from strong at low pressure to weak at high pressure as the system evolves from soft condensed matter to hard condensed matter. Implications are discussed on the development of stellar substance at the early stages of the Universe.

cond-mat.soft↗
arXiv2008

Off-center impurities in alkali halides: reorientation, electric polarization and binding to F center. V. Temperature-dependent electrostatic polarizabilities

We derive and discuss expressions for the temperature-dependent electrostatic polarizabilities of off-center ions holding good under various experimental conditions. At low temperatures and electric-field strengths, all of them reasonably reduce to values characteristic of phonon-coupled two-level systems. Prospects for further studies of the dispersive coupling are also considered.

cond-mat.mtrl-sci↗
arXiv2008

Famed Bulgarian physicists. I. St. Petroff's Goettingen research of the photostimulated interconversions of color centers in alkali halides: the discovery of the photostimulated aggregation

This essay tells briefly of the life and work of one of the most successful scientists originating from a Balkan settlement whose name and popularity have greatly exceeded its realm. The word is of a discovery during WWII of the photostimulated aggregation of the F centers (else alkali atoms) dissolved from the vapor into an alkali halide crystal. Using optical absorption techniques while a grantee of Humboldt's Foundation in Goettingen, Germany between 1943-1944, he found new absorption bands pertaining to small-size F center aggregates and followed their interconversions. A primary photochemical solid state reaction was evidenced for the first time leading to nanoscale products.

physics.hist-ph↗
arXiv2007

Vibronic polarons: comments on a model for the colossal field-resistance effects in manganites

In addition to mechanisms already proposed to account for the formation in manganites of a small-polaron superlattice above the Curie temperature Tc and to a metallic-like sea of large polarons below Tc, we now consider other observed colossal-resistance inducing fields, such as magnetic, electric, photon, or strain fields. We attribute the charge-ordered phase formation to the occurrence of strong dipolar binding of vibronic small polarons arising from the phonon coupling of highly polarizable two-level orbital systems. These species having associated inherent electric and magnetic off-center dipoles, they couple to the external fields leading to the observed colossal effects. The random phase appears due to polaron band widening in the external field.

cond-mat.mtrl-sci↗
arXiv2007

Colossal Van der Waals small-polaron superlattice: a hint to understanding the colossal magnetoresistance and electroresistance in manganites

A mechanism is proposed to explain the formation of a small-polaron superlattice above the Curie temperature in manganites. The order-disorder transition initiated by the external field at a lattice is known responsible for the colossal resistance effect. We attribute the lattice formation to the occurrence of strong Van der Waals pairing of vibronic small polarons arising from the phonon coupling of highly polarizable two-level orbital systems. The latter having inherent electric and magnetic dipoles associated with them, they couple to the external field leading to the observed colossal effects. We find numerical estimates of the critical field compatible.

physics.chem-ph↗
arXiv2007

Fine structures in the optical absorption spectra of photochemical silver in silver halides? A call for further research

A survey is presented of the work done so far to check earlier claims that a fine structure may be observed to occur under certain circumstances in the impurity spectral range of the optical absorption spectra of silver halides following photostimulation in the intrinsic range. This structure, associated with the photochemical formation of silver specks, has been questioned over the years. We now weigh carefully the experimental evidence on the silver halides against a background of similar data on the alkali halides, where competing processes run slower. We come to the conclusion that present day advances in experimental techniques may be quite adequate for providing a solid experimental basis to solve the problem unambiguously.

physics.chem-ph↗
arXiv2007

Nonradiative DKR processes: revisiting the theory. IV. On the controversy over a polaron state bound to an F center in alkali halides

We are commenting on an earlier hypothesis of polaron states bound to F centers in alkali halides. These states increasing the effective size of the color centers, they play an active role in concentration-dependent phenomena, such as the observed quenching of F center luminescence. Our record shows only one related study on NaBr and NaI which has also been aimed at checking the bound polaron hypothesis. Further studies of the concentration quenching in other alkali halide hosts whould eventually shine more light on the problem.

cond-mat.mtrl-sci↗
arXiv2007

Nonradiative DKR processes: revisiting the theory. III. Multimode approaches

We outline a method for dealing with the relaxation of optically excited NaI F centers in terms of a single effective frequency along a multimode coordinate. The 2s-2p mixing through coupling to a T_{1u} vibrational mode is also discussed with optimistic conclusions. Equilibrium nonradiative deexcitation rates are estimated and compared with intervibrational relaxations so as to assess the efficiency of the nonequilibrium DKR deexcitation. They show the former not to be competitive, due to the large number of phonons exchanged.

cond-mat.mtrl-sci↗
arXiv2007

Nonradiative DKR processes: revisiting the theory. II. Electron-vibrational mode coupling

We summarize a few proposals for mixing F center states through the mediation of an appropriate symmetry-breaking vibrational mode. Electron-mode coupling energies odd-order in the mode coordinates are characteristic of the pseudo-Jahn-Teller mixing of nearly-degenerate opposite-parity electronic states mediated by an odd-parity vibrational mode. Coupling energies even order in the vibrational coordinates lead to a pseudo-Renner mixing of even parity nearly-degenerate states or to dynamic-Renner mixing of degenerate states of whatever symmetry. Both Renner mixing energies add up to widen the crossover splitting of the adiabatic energies thereby enhancing the nonradiative deexcitation rate.

cond-mat.mtrl-sci↗
arXiv2007

Nonradiative DKR processes: revisiting the theory. I. Unimode model

We are revisiting the efficiency problem of DKR nonradiative transitions at local electronic centers in polarizable solids. Following a brief survey, we formulate the general rate equations thereby defining the microscopic parameters controlling the nonradiative deexcitation. We also reproduce the quantum mechanical expressions for the energy conserving "horizontal transfer rates" to compare with the "verical transition rates" at various vibronic energy levels.

cond-mat.mtrl-sci↗
arXiv2007

Quantum mechanical motion of off-center ion in external magnetic field

We consider the magnetostatic response to an external magnetic field of a crystal containing off-center ions, such as Li^+ in KCl and KBr or the apical oxygens O(A) in the LaSCO family of layered perovskites. Magnetic dipoles are deduced from the matrix elements of the energy operator of a spinning particle in a magnetic field which particle also satisfies the nonlinear Mathieu equation. The magnetic moments are found to increase in magnitude as the system goes from the lowest energy ground state to the higher lying excited states. Our conclusions are in concert with earlier studies of magnetic dipoles from circular currents based on Ampere's theorem.

cond-mat.mtrl-sci↗
arXiv2006

Reaction rates in squeezed polaron bands controlled by quantum statistics

Reaction rates are often defined using classical statistics for introducing the thermal occupation probabilities. Its predictions for the temperature dependence of a rate are found in reasonable agreement with experiments. In view of the applications to polaronic systems at lower temperatures under strongly quantized conditions, we now extend the definition so as to incorporate quantum statistics as well, Fermi-Dirac for polarons and Bose-Einstein for bipolarons. We find both extensions feasible.

cond-mat.mtrl-sci↗
arXiv2006

Two-electron F' centers in alkali halides: a saddle point approach. I. General and semicontinuum analyses

The F' center in an alkali halide forms when an anion vacancy traps two electrons which is the prerequisite of a diatomic molecule. Indeed, the center may displace left or right along <110> in a (110) plane, due to its coupling to the B_{1u} vibrational mode of <110> polarization respectively. On jumping from the initial position to the final position the F' center passes through a saddle point which configuration is molecule-like being conformed by two neighboring semi-vacancies along <110>. Each semi-vacancy traps one electron to change its effective charge from +1/2 to -1/2 formin a semi-F' center. We ouline the basic theory so as to perform calculations on the F' eigenenergies in the two configurations to see which one of them is more favorable.

cond-mat.mtrl-sci↗
arXiv2006

Vibronic solitons in a (nearly-) degenerate electronic system along a 1-D chain

We consider the formation and stability of the vibronic polaron arising in a degenerate or nearly degenerate electronic system coupled to an appropriate vibrational mode. We define the electron-phonon coupling as a mixing of the electronic states by the phonon mode. Our investigation is similar to that carried out recently by Dennis Clougherty though by introducing a different coupling operator. This extends the vibronic coupling range beyond the limits set by the difference in electronic density of the progenitor bands thereby widening the applicability range of the coupled soliton.

cond-mat.mtrl-sci↗
arXiv2005

Carbon diffusion in alpha-iron: Evidence for quantum mechanical tunneling

Recent experimental data on the diffusion coefficient of carbon in alpha-iron below liquid nitrogen temperature (LNT) question the classical approach to the observed temperature dependence. As the temperature is lowered below LNT, the diffusion constant tends to a nearly temperature-independent value rather than continuing its activated trend. The low temperature branch is apparently characteristic of a quantum mechanical process dominated by tunneling in ground state. Concommitantly, we apply an occurrence probability approach to describing the overall temperature dependence as a single continuous rate. Within the adiabatic approximation the electronic eigenvalue depending parametrically on the nuclear coordinates is taken to be the potential energy to control the motion of the nuclei. The resulting rate involves all horizontal-tunneling energy-conserving elastic transitions at the quantized energy levels of the migrating atom. A small though not negligible slope of in the temperature dependence as the temperature is raised below 100 K is dealt with by complementing for the rate of a parallel one-phonon inelastic tunneling process in excess to the basic elastic-tunneling rate. Our combined approach agrees well with the experimental data. In particular, the frequency of the coupled vibration is obtained virtually identical to the carbon vibrational frequency from inelastic neutron-scattering data. The migrational barrier is also found to be within the limits expected for alpha-iron.

cond-mat.mtrl-sci↗