Maia Elizbarashvili

Electromagnetic field-matter interaction in cavities gives rise to Rabi oscillations, which are used for numerous technologies such as atomic clocks, sensors, qubits, and computing. Our report is devoted to the almost completely classical consideration of free and forced Rabi oscillations in the coupled system "fictitious spins 1/2 + cavity" from the point of view of the general oscillation theory. Here, we limit ourselves to the case when the resonator and two-level system are tuned to each other. As a result of such consideration, the following results are obtained: i) normal frequencies and normal decays of free and forced Rabi oscillations are calculated; ii) at certain conditions level anticrossing can disappear, iii) the evolution of transfer function signals with changing spin-photon coupling is studied, and this evolution includes single-peak signals, the vicinity of the critical point, and double-peak signals. Our analytical results, presented also in the form of plots, agree with the experimental results of a number of studies where free Rabi oscillations were observed in both the frequency (polaritonic peaks) and time domains (damped output oscillation with the Rabi frequency). Since resonant oscillating external fields are used for manipulations of qubits, our "forced" results can be useful for it.  

Investigation of the spin-cavity polaritons (SCP) is important since they play a decisive role in creating long-storage quantum memories and optical interfaces, polaritonic chemistry, and masers, especially those which operate at room temperature, being of great significance for a lot of different applications. In this report, we suggest an almost completely classical model for the description of the polaritonic peaks conditioned both by free and forced Rabi oscillations of the SCP system. Our investigation is based on the linearized coupled differential equations we derived semi-classically for the magnetization component of the transition of the spin triplet states and the varying field in a cavity when the empty cavity is exactly tuned to the transition. The normal frequencies of this system of equations were found, and the corresponding graphs were constructed for the appropriate values of the parameters. It is shown that the increase in the spin-photon coupling can cause both the pushing apart of normal frequencies at a mutual decay rate (which is the known effect) and the merging together of normal decay rates at a mutual frequency (which is the predicted effect here). Polariton peaks, observable by the cavity transmission function and by EPR in the form of absorption and dispersion signals, were also investigated, and corresponding graphs were constructed. At that, instead of a transition of spin triplet states, a variety of other two-level systems (atoms, molecules, excitons, and so on) can act as emitters. Since the value of the spin-photon coupling is of great interest for applied scientists, obtaining this value by comparing our results with experimental data is of great importance

The purpose of this investigation is to clarify the following question: can the appearance of a limit cycle (LC) cause broadening of the line of stationary maser emission? To answer this question, we assumed that the maser output could affect the sample emitting it. This effect is similar to the effect on the emitting sample of an electromagnetic field from an external source, leading to Stark dynamic frequency shift (SDS). Using nonlinear dynamics methods, we carried out a bifurcation analysis of a nonlinear system of ordinary differential equations (ODE) describing massing on spin triplet states taking into account SDS. It was found that at a certain value of the detuning of the maser cavity frequency from the inverted transition frequency, the resulting system has an unstable saddle-node (SN) singular point with a stable nodal sector. With an arbitrarily small decrease in this detuning, SN bifurcation occurs ─ SN disappears and a stable LC appears. The angular frequency of this LC is calculated and its phase portrait is constructed to a first approximation using the well-known procedure for obtaining analytical periodic solutions of ODE system. It is shown that the presence of LC leads to the appearance of two harmonics in addition to the main line of maser emission. If they merge with it, they can widen the masing line in its central part. A similar broadening was observed in experimental work, where it was noted that it was caused by the appearance of LC. Since broadening of the maser output is an undesirable phenomenon, our results can help get rid of it

Maser Group of the Imperial College of London was able to use spin-triplet states (STSs) of photoexcited pentacene in p-terphenyl for the creation of a maser, which operated in zero constant magnetic field (ZF) at room temperature in pulse regime. Some of their experimental results were earlier interpreted by us, as a single pulse of the pure superradiance (the initial state of spins was assumed to be incoherent) from the coupled system "inverted transition of STS + resonator". At that, the homogeneous broadening of the inverted STS transition was supposed. The obtained analytical results were successfully applied for the comparison with the experiment. However, in a number of cases the EPR lines of STSs had signs of the inhomogeneous broadening (IB). Therefore, now the effect of the IB of the inverted transition of STSs on the character and parameters of the single-pulse pure superradiance (SR) from them is investigated in ZF. At that, it is assumed that the bandwidth of the resonator exceeds both the inhomogeneous linewidth of the STS transitions and the inverse value of the radiation damping time of the resonator with a sample. The dynamics of spins with the full zero field splitting of their levels is described with the help of the single transition operators. The coupling between spins and resonator is described semi-classically, as well as the motion of the full magnetization of a sample. The conditions for SR development from the "embryo" thermal photons and the time-domain shape of the SR pulse are obtained. Parameters of SR, monitored with the help of both the voltage and radiated power measurements are calculated: the pulse intensities, their delay times and widths. The measurable results of the paper can be of interest for investigators of masers using transitions of STSs and operating in ZF at room temperature.

The anisotropic dynamics of spin triplet states (STS) of molecular single crystals in zero constant and weak varying magnetic fields (weakness means the absence of saturation at the steady state and of the nutation at the pulse EPR) directed along the molecular axes, is analytically investigated. The equations are derived for the free motion of the sample magnetization, describing its linear oscillations along that molecular axis, along which its nonzero initial value was created. The tensor of the steady-state dynamical susceptibility to the varying field is found. The result of the action of a short MW pulse on STS is analytically described, containing a periodic dependence on the pulse duration and its detuning. Abovementioned results can be applied also to I=1 NMR, what is important for nitrogen-containing explosives and narcotics monitoring. The anisotropic dynamics of electron spin-lattice relaxation (SLR) at its one-phonon mechanism is investigated without the high temperature approximation over the phonon temperature; the SLR rates of the separate transitions of STS are calculated; the corresponding SLR probabilities are written in the form, which supposes the fractal dimensionality d of a lattice; the results with d=4/3 agree well with the experimental data in STS of the buried tryptophan of ribonuclease T1.

Some questions of the dynamics of the spin-triplet states (STS) are investigated. The equations of motion of the magnetization components related to the separate transitions of the EPR fine structure (FS) under the action of the constant and varying field are derived in the common case. Further consideration of the magnetization motion effects, including EPR, is restricted to the case of a non-zero constant field. It is demonstrated that at the definite condition fulfillment, the free motion of the sample magnetization after the excitation of one of the FS transitions is the precession at the frequency of the excited transition with an ellipse in the plane transverse to the constant field. At that, the square module of the magnetization vector contains as the constant part, so the part, oscillating at the double frequency of the excited transition. The tensor of the dynamic susceptibility of the spin-system (SS) to the microwave field is written at the creation of the resonance conditions for each transition of the well resolved FS. At the same condition, as above, the matrix elements of this tensor reflect the elliptical character of the magnetization precession at the stationary excitation by the transverse microwave field. Some aspects of the spin-lattice relaxation (SLR) of STS are analytically studied in the common case. The rate of the one-phonon SLR is obtained under conditions when SS is described by a single spin temperature. The SLR rates of the separate transitions of the FS are calculated at the same one-phonon mechanism. The low-frequency method of their finding is proposed with the help of the Gorter type experiment in the constant field, transforming the three-level SS into the two-level one. Three non-zero diagonal elements of the corresponding low-frequency susceptibility tensor are analytically obtained.

Some questions of the dynamics of the electron spins S=1 possessing the zero-field splitting are investigated in a strong constant magnetic field. The equations of motion of the magnetization components related to the separate transitions of the EPR fine structure (FS) are derived. It is demonstrated that the free motion of the sample magnetization is the precession at the frequency of the excited “allowed” transition with an ellipse in the plane transverse to the constant field, which is accompanied by the longitudinal magnetization component oscillation at the frequency of the “forbidden” transition. The tensor of the dynamic susceptibility of the spin-system (SS) to the microwave field is written at the creation of the resonance conditions for each transition of the well resolved FS. The matrix elements of this tensor reflect the elliptical character of the magnetization precession at the frequency of the very same transition, for which the resonance conditions are created. The SLR rates of the separate transitions of the FS are calculated at the same mechanism. The low-frequency method of their measurement is suggested with the help of the Gorter type experiment in a strong constant field transforming the three-level SS into the two-level one with the analytically derived expressions for the three non-zero diagonal elements of the low-frequency susceptibility tensor at the three directions of the constant field.

The EPR broadening model conditioned by the one-phonon mechanism of the anisotropic spin-spin reservoir spin-lattice relaxation in the weak constant field is suggested by us for the concentrated paramagnets with the dominating exchange interaction. It is shown in the given presentation that under the definite conditions the EPR linewidth by this model can be presented as the EPR linewidth by the Kubo-Tomita (KT) exchange narrowing theory, multiplied by the spin-phonon factor, proportional to the lattice temperature. The analytical results are reduced to the form suitable for the application to the obtaining of the anisotropic Dzyaloshinski-Moriya (DM) and crystal field (CF) interaction constants in materials with such interactions. The plots of the temperature and angular dependencies are built for the weak field EPR in the lightly doped lanthanum manganite (LM) single crystals agreeing with the experimental ones. The corresponding angular dependencies of the zero-field relaxation rates measurable in the Gorter type experiments are also plotted allowing at the availability of such experiments duplicating the EPR results. Тhe temperature and angular dependencies plotted according to the KT exchange narrowing theory with the subsequent account for the short range spin correlations in the case when the nature of the EPR broadening is of the pure spin-spin type (DM interaction) well agree with the experiment in KCuF3.

Nanotechnology is defined as the manipulation of matter with at least one dimension sized from 1 to 100 nanometers. Although modern semiconductors manufactured are measured in nanometers, production of semiconductors is not traditionally classed as nanotechnology. Nanotechnology is likely to manifest itself in the semiconductor industry as semiconductor devices. The examples of modern semiconductor technology are quantum dots, quantum wires or nanotubes and quantum wells. The physical properties and various methods of fabrication III – V semiconductor QD’s are described early. Because of its dimensions nanostructured materials are useful to interact with light, and it is thought that twenty-one century will be the era of photons. Although one use of III – V semiconductor QD’s is high effective light emitted devices, as well as and wavelength conversion changing emitted light to the desired spectrum, the resent works show that two main field of application QD are: a new class of fluorescent materials for biosensor, and energetic materials for new generation solar cells. The method of electrochemical deposition of metals (In, Ga, Al, Sb, Bi, Cu, Ni, Ag, Pt, Pd, Fe) on semiconductor surface was used for fabrication of various semiconductor devices. By deposition of the III group metals (In, Ga) on III – V semiconductor GaP, followed with heat treatment in hydrogen, it was obtained the nanostructured layer InxGa1–xP on GaP surface, and the possibility of application of obtained structures for quantum dot solar cell was theoretically investigated. The current research is an attempt of obtaining spintronic material by original method of electrochemical deposition of ferromagnetic metals on the GaAs surface and investigation of their

electrical and photoelectric properties. The process steps are improved and novel process techniques are developed for manufacturing of GaAs-based devices.

Analytical expressions are derived for the rates of longitudinal and transverse nuclear spin relaxation under conditions of fast modulation of the magnitude and direction of a hyperfine field induced by unpaired electrons of an ion. The results obtained are used to explain the data available in the literature on the 55Mn spin relaxation in the ferromagnetic metallic phase of doped perovskites, in which the modulation of the hyperfine field is caused by the hopping of eg electrons between Mn³⁺  and Mn⁴⁺ ions. It is demonstrated that, within this model, the rates of longitudinal and transverse relaxation are characterized by the same temperature dependence and their ratio is independent of temperature, which is in agreement with the experimental data.

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