Sub-nanosecond delay of light in (Cd,Zn)Te crystal

We study excitonic polariton relaxation and propagation in bulk CdZnTe using time- resolved photoluminescence and time-of-flight techniques. Propagation of picosecond optical pulses through 0.745 mm thick crystal results in time delays up to 350 ps, depending on the photon energy. Optical pulses with 150 fs duration become strongly stretched. The spectral dependence of group velocity is consistent with the dispersion of the lower excitonic polariton branch. The lifetimes of excitonic polariton in the upper and lower branches are 1.5 and 3 ns, respectively.Comment: 5 pages, 4 figure

Weak and strong coupling limits of the two-dimensional Fr\"ohlich polaron with spin-orbit Rashba interaction

The continuous progress in fabricating low-dimensional systems with large spin-orbit couplings has reached a point in which nowadays materials may display spin-orbit splitting energies ranging from a few to hundreds of meV. This situation calls for a better understanding of the interplay between the spin-orbit coupling and other interactions ubiquitously present in solids, in particular when the spin-orbit splitting is comparable in magnitude with characteristic energy scales such as the Fermi energy and the phonon frequency. In this article, the two-dimensional Fr\"ohlich electron-phonon problem is reformulated by introducing the coupling to a spin-orbit Rashba potential, allowing for a description of the spin-orbit effects on the electron-phonon interaction. The ground state of the resulting Fr\"ohlich-Rashba polaron is studied in the weak and strong coupling limits of the electron-phonon interaction for arbitrary values of the spin-orbit splitting. The weak coupling case is studied within the Rayleigh-Schr\"odinger perturbation theory, while the strong-coupling electron-phonon regime is investigated by means of variational polaron wave functions in the adiabatic limit. It is found that, for both weak and strong coupling polarons, the ground state energy is systematically lowered by the spin-orbit interaction, indicating that the polaronic character is strengthened by the Rashba coupling. It is also shown that, consistently with the lowering of the ground state, the polaron effective mass is enhanced compared to the zero spin-orbit limit. Finally, it is argued that the crossover between weakly and strongly coupled polarons can be shifted by the spin-orbit interaction.Comment: 11 pages, 5 figure

Correlation of optical conductivity and ARPES spectra of strong-coupling large polarons and its display in cuprates

Common approach is used to calculate band due to strong-coupling large polaron (SCLP) photodissociation in ARPES and in optical conductivity (OC) spectra. It is based on using the coherent-states representation for the phonon field in SCLP. The calculated positions of both band maximums are universal functions of one parameter - the SCLP binding energy Ep: ARPES band maximum lies at binding energy about 3.2Ep; the OC band maximum is at the photon energy about 4.2Ep. The half-widths of the bands are mainly determined by Ep and slightly depend on Frohlich electron-phonon coupling constant: for its value 6-8 the ARPES band half-width is 1.7-1.3Ep and the OC band half-width is 2.8-2.2Ep. Using these results one can predict approximate position of ARPES band maximum and half-width from the maximum of mid-IR OC band and vice versa. Comparison of the results with experiments leads to a conclusion that underdoped cuprates contain SCLPs with Ep=0.1-0.2 eV that is in good conformity with the medium parameters in cuprates. The values of the polaron binding energy determined from experimental ARPES and OC spectra of the same material are in good conformity too: the difference between them is within 10 percent.Comment: 17 pages, 6 figure

Directed current in the Holstein system

We propose a mechanism to rectify charge transport in the semiclassical Holstein model. It is shown that localised initial conditions, associated with a polaron solution, in conjunction with a nonreversion symmetric static electron on-site potential constitute minimal prerequisites for the emergence of a directed current in the underlying periodic lattice system. In particular, we demonstrate that for unbiased spatially localised initial conditions, violation of parity prevents the existence of pairs of counter-propagating trajectories, thus allowing for a directed current despite the time-reversibility of the equations of motion. Occurrence of long-range coherent charge transport is demonstrated

A Theoretical Model for Gas Separation in a Glow Discharge: Cataphoresis

A theoretical model for transient and steady-state cataphoresis is developed starting with the macroscopic equations of continuity. After a brief breakdown period, the impurity ions are assumed to be closely coupled with their neutral counterparts. The basic assumptions in the model are that after breakdown, the level of ionization of the impurity, and the axial electric field remain constant; it is demonstrated that under these conditions a system involving rapid ionization-recombination reactions is equivalent to a system in which no reaction occurs, but in which the effective\u27\u27 ion mobility is a product of the true ion mobility and the fraction of impurity ionization. The influence of endbulbs commonly employed in experiments is analyzed and found to influence greatly the characteristic time required to reach steady state. Agreement is found between the model and available experimental data. Particular emphasis is placed upon mass spectrometer data reported by Matveeva, and by Beckey, Groth, and Welge; these data are for mixtures of rare gases and for mixtures of hydrogen and deuterium, and involve endbulbs. The ordinary diffusion case, associated with the collapse of the steady-state cataphoretic profile, is also analyzed for a system containing endbulbs

Optical properties of atomic Mott insulators: from slow light to dynamical Casimir effects

We theoretically study the optical properties of a gas of ultracold, coherently dressed three-level atoms in a Mott insulator phase of an optical lattice. The vacuum state, the band dispersion and the absorption spectrum of the polariton field can be controlled in real time by varying the amplitude and the frequency of the dressing beam. In the weak dressing regime, the system shows unique ultra-slow light propagation properties without absorption. In the presence of a fast time modulation of the dressing amplitude, we predict a significant emission of photon pairs by parametric amplification of the polaritonic zero-point fluctuations. Quantitative considerations on the experimental observability of such a dynamical Casimir effect are presented for the most promising atomic species and level schemes

Polarons and slow quantum phonons

We describe the formation and properties of Holstein polarons in the entire parameter regime. Our presentation focuses on the polaron mass and radius, which we obtain with an improved numerical technique. It is based on the combination of variational exact diagonalization with an improved construction of phonon states, providing results even for the strong coupling adiabatic regime. In particular we can describe the formation of large and heavy adiabatic polarons. A comparison of the polaron mass for the one and three dimensional situation explains how the different properties in the static oscillator limit determine the behavior in the adiabatic regime. The transport properties of large and small polarons are characterized by the f-sum rule and the optical conductivity. Our calculations are approximation-free and have negligible numerical error. This allows us to give a conclusive and impartial description of polaron formation. We finally discuss the implications of our results for situations beyond the Holstein model.Comment: Final version, 10 pages, 10 figure

Electron-lattice interaction and its impact on high Tc superconductivity

In this Colloquium, the main features of the electron-lattice interaction are discussed and high values of the critical temperature up to room temperature could be provided. While the issue of the mechanism of superconductivity in the high Tc cuprates continues to be controversial, one can state that there have been many experimental results demonstrating that the lattice makes a strong impact on the pairing of electrons. The polaronic nature of the carriers is also a manifestation of strong electron-lattice interaction. One can propose an experiment that allows an unambiguous determination of the intermediate boson (phonon, magnon, exciton, etc.) which provides the pairing. The electron-lattice interaction increases for nanosystems, and this is due to an effective increase in the density of states

Effect of electron-phonon interaction range on lattice polaron dynamics: a continuous-time quantum Monte Carlo study

We present the numerically exact ground state energy, effective mass, and isotope exponents of a one-dimensional lattice polaron, valid for any range of electron-phonon interaction, applying a new continuous-time Quantum Monte Carlo (QMC) technique in a wide range of coupling strength and adiabatic ratio. The QMC method is free from any systematic finite-size and finite-time-step errors. We compare our numerically exact results with analytical weak-coupling theory and with the strong-coupling $1/\lambda$ expansion. We show that the exact results agree well with the canonical Fr\"ohlich and Holstein-Lang-Firsov theories in the weak and strong coupling limits, respectively, for any range of interaction. We find a strong dependence of the polaron dynamics on the range of interaction. An increased range of interaction has a similar effect to an increased (less adiabatic) phonon frequency: specifically, a reduction in the effective mass.Comment: 27 pages, 16 figures, to appear Phys Rev B. Introduction rewritten, comparison with other authors extended, description of method shortened, improved treatment of weak coupling theor

Polarons as stable solitary wave solutions to the Dirac-Coulomb system

We consider solitary wave solutions to the Dirac--Coulomb system both from physical and mathematical points of view. Fermions interacting with gravity in the Newtonian limit are described by the model of Dirac fermions with the Coulomb attraction. This model also appears in certain condensed matter systems with emergent Dirac fermions interacting via optical phonons. In this model, the classical soliton solutions of equations of motion describe the physical objects that may be called polarons, in analogy to the solutions of the Choquard equation. We develop analytical methods for the Dirac--Coulomb system, showing that the no-node gap solitons for sufficiently small values of charge are linearly (spectrally) stable.Comment: Latex, 26 page