Interaction of strongly correlated electrons and acoustical phonons

We investigate the interaction of correlated electrons with acoustical phonons using the extended Hubbard-Holstein model in which both, the electron-phonon interaction and the on-site Coulomb repulsion are considered to be strong. The Lang-Firsov canonical transformation allows to obtain mobile polarons for which a new diagram technique and generalized Wick's theorem is used. This allows to handle the Coulomb repulsion between the electrons emerged into a sea of phonon fields (\textit{phonon clouds}). The physics of emission and absorption of the collective phonon-field mode by the polarons is discussed in detail. Moreover, we have investigated the different behavior of optical and acoustical phonon clouds when propagating through the lattice. In the strong-coupling limit of the electron-phonon interaction, and in the normal as well as in the superconducting phase, chronological thermodynamical averages of products of acoustical phonon-cloud operators can be expressed by one-cloud operator averages. While the normal one-cloud propagator has the form of a Lorentzian, the anomalous one is of Gaussian form and considerably smaller. Therefore, the anomalous electron Green's functions can be considered to be more important than corresponding polarons functions, i.e., pairing of electrons without phonon-clouds is easier to achieve than pairing of polarons with such clouds.Comment: : 28 pages, 9 figures, revtex4. Invited paper for a special issue of Low Temperature Physics dedicated to the 20th anniversary of HTS

Diagrammatic theory for Periodic Anderson Model: Stationary property of the thermodynamic potential

Diagrammatic theory for Periodic Anderson Model has been developed, supposing the Coulomb repulsion of $f-$ localized electrons as a main parameter of the theory. $f-$ electrons are strongly correlated and $c-$ conduction electrons are uncorrelated. Correlation function for $f-$ and mass operator for $c-$ electrons are determined. The Dyson equation for $c-$ and Dyson-type equation for $f-$ electrons are formulated for their propagators. The skeleton diagrams are defined for correlation function and thermodynamic functional. The stationary property of renormalized thermodynamic potential about the variation of the mass operator is established. The result is appropriate as for normal and as for superconducting state of the system.Comment: 12 pages, 10 figure

Detecting Bose-Einstein condensation of exciton-polaritons via electron transport

We examine the Bose-Einstein condensation of exciton-polaritons in a semiconductor microcavity via an electrical current. We propose that by embedding a quantum dot p-i-n junction inside the cavity, the tunneling current through the device can reveal features of condensation due to a one-to-one correspondence of the photons to the condensate polaritons. Such a device can also be used to observe the phase interference of the order parameters from two condensates.Comment: 5 Pages, 3 Figure

КОНЦЕНТРАЦІЯ ЯК ФАКТОР ФОРМУВАННЯ ЕКОНОМІЧНОЇ ЕФЕКТИВНОСТІ ВИРОБНИЦТВА МОЛОКА В СІЛЬСЬКОГОСПОДАРСЬКИХ ПІДПРИЄМСТВАХ

The purpose of the research is to assess the impact of the level of production concentration in dairy farming on its efficiency. The study was conducted on the example of agricultural enterprises of Kharkiv and Chernihiv regions of Ukraine. Theoretical provisions concerning the influence of production concentration on various aspects of efficiency were generalized in the article. It is emphasized that this process is objective and takes place in different countries. The advantages of large milk production enterprises are in greater opportunities for the use of technical means and savings at the level of fixed costs.The study has found that there is a clear relationship between the grouping criterion and the value of profitability, productivity of cows, based on the analysis of factual data on the results of agricultural enterprises grouping as to the volumes of milk sale. This dependence is direct. In addition, the relationship between the grouping indicator and the growth of milk production per 100 hectares of agricultural land, production intensity is quite clearly stated. In Kharkiv region, these dependencies are clearer. The scientific value of the research results lies in their practical orientation, considering the established dependencies when planning both the level of concentration, and milk production intensity in agricultural enterprises.Метою наукових досліджень є оцінка впливу рівня концентрації виробництва в молочному скотарстві на його ефективність. Дане дослідження проводилось на прикладі сільськогосподарських підприємств Харківської та Чернігівської області України. Було здійснено узагальнення теоретичних положень відносно впливу концентрації виробництва на різні аспекти ефективності. Підкреслюється, що даний процес має об’єктивний характер і відбувається в різних країнах світу. Переваги великих підприємств з виробництва молока обумовлені більшими можливостями з використання технічних засобів та економії на рівні постійних витрат. На основі проведеного дослідження було встановлено, що на підставі аналізу фактичних даних за результатами групування сільськогосподарських підприємств за критерієм обсягів реалізації молока має місце чітка залежність між групуючим критерієм та величиною рентабельності, продуктивності корів. Дана залежність має прямий характер. Крім того, доволі чітко констатується залежність між групуючим показником та зростанням обсягів виробництва молока на 100 га сільськогосподарських угідь, рівнем інтенсивності виробництва. В Харківській області дані залежності мали більш чіткий характер. Наукова цінність результатів досліджень полягає в їх практичному спрямуванні та можливості врахування встановлених залежностей при планування як рівня концентрації, так і рівня інтенсивності виробництва молока в сільськогосподарських підприємствах

Excitonic BCS-BEC crossover at finite temperature: Effects of repulsion and electron-hole mass difference

The BCS to Bose-Einstein condensation (BEC) crossover of electron-hole (e-h) pairs in optically excited semiconductors is studied using the two-band Hubbard model with both repulsive and attractive interactions. Applying the self-consistent t-matrix approximation combined with a local approximation, we examine the properties of a normal phase and an excitonic instability. The transition temperature from the normal phase to an e-h pair condensed one is studied to clarify the crossover from an e-h BCS-like state to an excitonic Bose-Einstein condensation, which takes place on increasing the e-h attraction strength. To investigate effects of the repulsive interaction and the e-h mass difference, we calculate the transition temperature for various parameters of the interaction strengths, the e-h particle density, and the mass difference. While the transition temperature in the e-h BCS regime is sufficiently suppressed by the repulsive interaction, that of the excitonic BEC is largely insensitive to it. We also show quantitatively that in the whole regime the mass difference leads to large suppression of the transition temperature.Comment: 8 pages, 7 figures, to be published in Phys. Rev.

Hadronic Gamma Rays from Supernova Remnants

A gas cloud near a supernova remnant (SNR) provides a target for pp-collisions leading to subsequent gamma-ray emission through neutral pion decay. The assumption of a power-law ambient spectrum of accelerated particles with index near -2 is usually built into models predicting the spectra of very-high energy (VHE) gamma-ray emission from SNRs. However, if the gas cloud is located at some distance from the SNR shock, this assumption is not necessarily correct. In this case, the particles which interact with the cloud are those leaking from the shock and their spectrum is approximately monoenergetic with the injection energy gradually decreasing as the SNR ages. In the GLAST energy range the gamma-ray spectrum resulting from particle interactions with the gas cloud will be flatter than expected, with the cutoff defined by the pion momentum distribution in the laboratory frame. We evaluate the flux of particles escaping from a SNR shock and apply the results to the VHE diffuse emission detected by the HESS at the Galactic centre.Comment: 4 pages, 3 figures. Contribution to the 30th ICRC, Merida, Mexico, 2007 (final version

Research On Maximizing Critical And Reducing Initial Heat Flux Densities To Eliminate Any Film Boiling And Minimize Distortion During Quenching

In the paper the results of testing three types of FUCHS oils: Thermisol QH 120, Thermisol QH 10 and Thermisol QB 46 are discussed. The main attention is paid to critical heat flux densities evaluation because they create a basis for optimizing cooling intensity of any liquid quenchant. In the paper is underlined that any film boiling during quenching is undesirable since it is a reason for big distortion and non-uniform surface harness. It is shown that intensive quenching decreases distortion of steel parts during quenching. To eliminate film boiling during quenching in mineral oils, optimal temperature of oil should be chosen which maximize the first critical heat flux density and special additives should be used to decrease initial heat flux by creating surface micro-coating. Along with the evaluation of heat transfer coefficients, critical heat flux densities inherent to liquid quenchant must be measured first to optimize quenching processes. International DATABASE on cooling characteristics of liquid quenchants must include critical heat flux densities, initial heat flux densities, and heat transfer coefficients allowing optimizing and governing quenching processes

Two-band superconductors: Hidden criticality deep in the superconducting state

We show that two-band superconductors harbor hidden criticality deep in the superconducting state, stemming from the critical temperature of the weaker band taken as an independent system. For sufficiently small interband coupling $\gamma$ the coherence length of the weaker band exhibits a remarkable deviation from the conventional monotonic increase with temperature, namely, a pronounced peak close to the hidden critical point. The magnitude of the peak scales proportionally to \gamma^(-\mu), with the Landau critical exponent \mu = 1/3, the same as found for the mean-field critical behavior with respect to the source field in ferromagnets and ferroelectrics. Here reported hidden criticality of multi-band superconductors can be experimentally observed by, e.g., imaging of the variations of the vortex core in a broader temperature range. Similar effects are expected for the superconducting multilayers.Comment: 6 pages, 2 figures, Supplementary material included. Accepted for publication in PR

Very-High-Energy Gamma-Ray Signal from Nuclear Photodisintegration as a Probe of Extragalactic Sources of Ultrahigh-Energy Nuclei

It is crucial to identify the ultrahigh-energy cosmic-ray (UHECR) sources and probe their unknown properties. Recent results from the Pierre Auger Observatory favor a heavy nuclear composition for the UHECRs. Under the requirement that heavy nuclei survive in these sources, using gamma-ray bursts as an example, we predict a diagnostic gamma-ray signal, unique to nuclei - the emission of de-excitation gamma rays following photodisintegration. These gamma rays, boosted from MeV to TeV-PeV energies, may be detectable by gamma-ray telescopes such as VERITAS, HESS, and MAGIC, and especially the next-generation CTA and AGIS. They are a promising messenger to identify and study individual UHE nuclei accelerators.Comment: 7 pages, 4 figures, accepted for publication in PRD, with extended descriptions. Conclusions unchange