On the nature of coherent phonons generated by ultrashort laser pulses in single-crystal antimony

Reflectivity oscillations generated by A_1g coherent phonons in an antimony single crystal have been studied by a method involving pumping and probing by femtosecond laser pulses, which was complemented by spectral filtration of the signal. An analysis of the spectrally resolved signal showed that not only the integrated intensity but also the spectrum of the probe pulse are functions of the delay time between the pumping and probing and oscillate between the Stokes and anti-Stokes components at the optical-phonon frequency. A comparison of the integrated lattice excitation relaxation dynamics with the spectrally resolved lattice excitation relaxation dynamics revealed new facets in the nature and generation mechanism of coherent phonons.     

Pump pulse duration dependence of coherent phonon amplitudes in antimony

Coherent optical phonons of A_1k and E_k symmetry in antimony have been studied using the femtosecond pump–probe technique. By varying the pump-pulse duration and keeping the probe duration constant, it was shown that the amplitude of coherent phonons of both symmetries exponentially decreases with increasing pulse width. It was found that the amplitude decay rate for the fully symmetric phonons with larger frequency is greater than that of the doubly degenerate phonons, whereas the frequency and lifetime for coherent phonons of both symmetries do not depend on the pump-pulse duration. Based on this data, the possibility of separation between dynamic and kinematic contributions to the generation mechanism of coherent phonons is discussed.     

Observation of coherent optical phonons excited by femtosecond laser radiation in Sb films by ultrafast electron diffraction method

The generation of coherent optical phonons in a polycrystalline antimony film sample has been investigated using femtosecond electron diffraction method. Phonon vibrations have been induced in the Sb sample by the main harmonic of a femtosecond Ti:Sa laser (λ = 800 nm) and probed by a pulsed ultrashort photoelectron beam synchronized with the pump laser. The diffraction patterns recorded at different times relative to the pump laser pulse display oscillations of electron diffraction intensity corresponding to the frequencies of vibrations of optical phonons: totally symmetric (A _1g ) and twofold degenerate (E _g ) phonon modes. The frequencies that correspond to combinations of these phonon modes in the Sb sample have also been experimentally observed.     

Enhancement of waves at rotational oscillations of a liquid

The generation of coherent optical phonons in an antimony film has been directly observed by the femtosecond electron diffraction method. The sample has been excited by a femtosecond laser pulse (λ = 800 nm) and probed with a pulsed photoelectron beam. Oscillations of the intensity corresponding to vibration frequencies of optical phonons excited by the laser have been observed in the obtained diffraction patterns: totally symmetric (A_1g) and twofold degenerate (E_2g) phonon modes of antimony and their combinations.     

Phonon autoecho in bismuth and antimony single crystals

Phonon autoecho is observed upon pumping Bi and Sb semimetals with ultrashort high-energy laser pulses. The autoecho is manifested as a revival of reflection oscillations generated by an A_1g coherent phonon after their complete disappearance. The phenomenon of phonon autoecho offers decisive evidence of the nonclassical character of the state of the crystal lattice that is accomplished in pumping-probing experiments by femtosecond laser pulses.     

Coherent control of the lattice dynamics of bismuth near the lindemann stability limit

The relative contributions of the anharmonicity of the lattice potential and the nonequilibrium concentration of charge carriers to the time dependence of the coherent A _1g phonon frequency in bismuth excited by high-energy ultrashort laser pulses are studied by the coherent control method. The contributions are separated by the pump-probe method in which excitation is performed by two pulses with a controlled time delay. It is shown that, depending on the relative delay between the pump pulses, both correlation and anticorrelation between the amplitude and the initial frequency of oscillations are observed while the chirp and the initial frequency of the coherent phonon are anticorrelated. In addition, it has been found that the contributions of the lattice and the electronic subsystem are always anticorrelated. Therefore, a certain phase can be assigned to an electronic excitation and it may be suggested that the time dependence of the phonon frequency is determined not only by instantaneous values of the lattice and electronic response but also by the phase relations between them.     

Variable Temperature Single Crystal EPR Studies of Cu(II) in Dipotassium Diaquabis(Malonato-<Emphasis Type="Italic">κ</Emphasis><Superscript>2</Superscript><Emphasis Type="Italic">O,O</Emphasis>′) Nickelate Dihydrate: An Example of Contaminated Ground State

Single crystal X-band electron paramagnetic resonance (EPR) studies on divalent copper ions embedded in dipotassium diaquabis(malonato-κ ² O,O′) nickelate dihydrate have been performed at 300, 123 and 77 K to understand the nature of Jahn–Teller distortion in the paramagnetic host lattice. The angular variation of the EPR spectra reveals the presence of two sites, with one site not showing hyperfine resolution even at 77 K. The spin-Hamiltonian parameters of this six-coordinated Cu(II) ion, evaluated from EPR spectra at various temperatures, are:

• 300 K: g ₁₁ = 2.125, g ₂₂ = 2.118, g ₃₃ = 2.290, no copper hyperfine resolution
• 123 K: g ₁₁ = 2.229, g ₂₂ = 2.113, g ₃₃ = 2.319 and A ₁₁ = 5.02, A ₂₂ = 3.82, A ₃₃ = 6.87 mT
• 77 K: g ₁₁ = 2.224, g ₂₂ = 2.114, g ₃₃ = 2.324 and A ₁₁ = 5.32, A ₂₂ = 3.90, A ₃₃ = 7.06 mT

respectively. The low value observed for A ₃₃ at 123 and 77 K has been explained by assuming a ground state \({\text{d}}_{{x^{2} - y^{2} }}\) wave function for Cu(II) ions, contaminated with the excited state \({\text{d}}_{{z^{2} }}\). From the temperature dependence of the EPR spectra, the Cu(II) ions can be considered as a static Jahn–Teller system, with contaminated ground state. The admixture coefficients and bonding parameters have also been calculated by combining EPR and optical data. The EPR spectrum of powder sample confirms single crystal data.

     

Multiphoton intraband absorption of femtosecond light pulses in crystals: II. Processes with the participation of acoustic and optical phonons

For arbitrary integers n, we have calculated probabilities P _n of n-photon intraband transitions with the participation of longitudinal acoustic and longitudinal optical phonons. Dependences of P _n on the duration of femtosecond pulses have been obtained. These dependences differ from those that are observed in the case of quasisteady electromagnetic radiation.     

Die Bewegung eines Exzitons entlang eines Polymers unter dem Einfluß der Gitterschwingungen

In this paper the influence of lattice vibrations on the migration of electronic excitation energy along a one-dimensional system is treated with the aid of perturbation theory. The lattice is assumed to be in thermodynamic equilibrium, so that it is possible to average over all the lattice coordinates. With this assumption the caseT=0 is solvable exactly; the propagation of energy will be coherent. The caseT>0, however, has to be treated with an approximation that becomes invalid for very low temperatures. It results that atT>0 the exponentially decreasing coherent part of the propagation is accompanied by an incoherent part, which, after a certain critical timet _kr, becomes the more important one;t _kr is a function of the parameters that specify the system.     

Influence of the Axial-Vector Coupling Constant and the Energy Distribution Function on <Emphasis Type="Italic">β</Emphasis>-Decay Rates Within the Gross Theory of Beta Decay

We evaluate the β-decay rates within the gross theory of beta decay (GTBD) and compare the results for different values of the axial-vector coupling constant, g_A =?0.76, g_A =?0.88, g_A =?1, g_A =?1.13, and g_A =?1.26, and also different energy distribution functions like Gaussian, exponential, Lorentzian, and modified Lorentzian ones. We use new sets of parameters as well as updated experimental mass defects and also an improved approximation for the Fermi function. We compare our calculated results for a set of 94 nuclei of interest in pre-supernova phase, with experimental data in terrestrial conditions and also with other theoretical models like the QRPA, the shell model (SM), and different versions of the GTBD. We show that best results are obtained with g_A =?1 using Gaussian and Lorentzian distributions, being the rates for the 74 and 80% of our sample, respectively, of the same order of magnitude that of experimental data. Finally, we show that the present results within the GTBD are better than those within the QRPA model and also older versions of the GTBD for the isotopes of cobalt and iron families, and comparable with SM for some elements.     

Magnetic phase diagrams of manganites in the electron doping region

The paper reviews the physical properties of the R_1?xA_xMnO₃ manganites (R=La, Pr, Nd, Sm, etc., A=Ca, Sr, Ba) in the region of their electron doping where the divalent atom concentration x in the compound lies in the interval 0.5<x<1.0. Experimental magnetic phase diagrams of the most well-studied compounds and the results of theoretical calculations of these diagrams made in the tight-binding approximation within the degenerate double-exchange model for T=0 are presented. The experimental section of the review deals primarily with neutron diffraction studies of the magnetic and crystal structures of the manganites, and the theoretical part, with the relation between their magnetic and orbital structures. The review describes, in considerable detail, the method of calculation of the energy spectrum ?(k) and of the total carrier energy for all possible magnetic and orbital configurations of the system corresponding to the translation symmetry of the lattice. The theoretical analysis is carried out separately for two models of the crystal structure, with two and four manganese atoms in the unit cell. All equilibrium magnetic and orbital configurations of the four-sublattice manganite model were determined by minimizing the total energy of the system with respect to the directions of the local manganese magnetic moments and orbital states of the e_g electrons. It is shown that, by using the effective Hamiltonian of the degenerate double-exchange model for the e_g electrons, which takes into account the e_g level splitting, and the Heisenberg Hamiltonian of the localized t_2g electrons, one can describe the diversity of the magnetic phases, the sequence of their alternation with increasing x, and the correlation between the spin and orbital degrees of freedom, which are observed in most electron-doped manganites.     

Coherent phonons in NdBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> single crystals: Optical-response anisotropy and hysteretic behavior

Femtosecond pump-probe measurements of reflection from crystallographic planes are performed to investigate lattice relaxation dynamics in the NdBa₂Cu₃O_{7? x} high-temperature superconductor. Ultrafast phonon response is examined over a wide temperature range for various orientations of the pump and probe polarization vectors with respect to particular crystallographic axes. The initial phases of coherent phonons are measured, and hysteretic behavior is revealed in the transition between two temperature regions above T_c for the ac plane.     

The effect of high pressure on the structure and on the magnetic and electronic properties of nickel monoxide

A diamond anvil cell is used to investigate the effect of high pressure (up to 37.5 GPa) on the optical absorption spectra of a single crystal of nickel oxide (NiO). In addition, strain-gage measurements are used to experimentally investigate the V(P) equation of state at a hydrostatic pressure of up to 8.5 GPa in a high-pressure chamber of the “toroid” type. Measurements are performed at room temperature. Absorption bands are observed, which correspond to optical d-d transitions of Ni²⁺ ion in the crystal field of ligands ³A_2g → ³T_2g, ³A_2g → {au1}E_1g, ³A_2g → ³T_1g(F), and ³A_2g → ¹T_2g. The values of energy of these transitions increase linearly with pressure, and their pressure coefficients are 7.3 ± 0.2, 2.87 ± 0.9, 9.7 ± 0.5, and 8.9 ± 0.3 meV/GPa, respectively. The pressure derivative of the crystal field parameter 10Dq corresponding to the ³A_2g → ³T_2g transition gives the pressure dependence of the magnitude of exchange integral J in the Anderson hybridization model. It is found that, in the pressure range from zero to 37.5 GPa, the behavior of the exchange integral J is largely defined by the hybridization parameter b = (10Dq/3). At the same time, the Coulomb interaction parameter U_eff is independent of pressure and, therefore, has no effect on the variation of J. The Coulomb interaction U_eff ≈ 7.47 ± 0.005 eV is determined. The experimental data on the equation of state are used to derive the \(J \propto V^\varepsilon \) correlation, where ε = ?2.99 ± 0.15, which is in good agreement with the predictions of Bloch’s theory (ε = ?10/3).     

Percolation and the electron–electron interaction in an array of antidots

A square lattice of microcontacts with a period of 1 μm in a dense low-mobility two-dimensional electron gas is studied experimentally and numerically. At the variation of the gate voltage V _g , the conductivity of the array varies by five orders of magnitude in the temperature range T from 1.4 to 77 K in good agreement with the formula σ(V _g ) = (V _g ?V _g ^* (T))^β with β = 4. The saturation of σ(T) at low temperatures is absent because of the electron–electron interaction. A random-lattice model with a phenomenological potential in microcontacts reproduces the dependence σ(T, V _g ) and makes it possible to determine the fraction of microcontacts x(V _g , T) with conductances higher than σ. It is found that the dependence x(V _g ) is nonlinear and the critical exponent in the formula σ ∝ ? (x - 1/2) ^t in the range 1.3 < t(T, V _g ) < β.     

Stable branches of a solution for a fermion on domain wall

The case when a fermion occupies an excited nonzero frequency level in the field of domain wall is discussed. It is demonstrated that a solution exists for the coupling constant in the limited interval 1 < g < g _max ≈ 1.65. It is shown that indeed there are different branches of stable solution for g in this interval. The first one corresponds to a fermion located on the domain wall (\(1 < g < \sqrt[4]{{2\pi }}\)). The second branch, which belongs to the interval \(\sqrt[4]{{2\pi }} \leqslant g \leqslant g_{\max }\), describes a polarized fermion off the domain wall. The third branch with 1 < g < g _max describes an excited antifermion in the field of the domain wall.     

Population Dynamics of Excited Atoms in Dissipative Cavities

Population dynamics of excited atoms in dissipative cavities is investigated in this work. We present a method of controlling populations of excited atoms in dissipative cavities. For the initial state |ee〉_AB|00〉_ab, the repopulation of excited atoms can be obtained by using atom-cavity couplings and non-Markovian effects after the atomic excited energy decays to zero. For the initial state |gg〉_AB|11〉_ab, the two atoms can also be populated to the excited states from the initial ground states by using atom-cavity couplings and non-Markovian effects. And the stronger the atom-cavity coupling or the non-Markovian effect is, the larger the number of repopulation of excited atoms is. Particularly, when the atom-cavity coupling or the non-Markovian effect is very strong, the number of repopulation of excited atoms can be close to one in a short time and will tend to a steady value in a long time.     

Order–order transition structural state in titanium monoxide TiO<Subscript>1.0</Subscript>

A new class of defect structures in which point defects of a crystal lattice simultaneously occupy sites of two different superstructures is proposed. The formation of these structural modifications is due to a second-order order–order phase transition that does not occur to the end. The allowable relation between the long-range order parameters in the structural modification formed by a combination of the monoclinic (M ₅ X ₅)_mon (space group C2/m (A2/m)) and the cubic (M ₅ X ₅)_cub (space group Pm3m) of the superstructures is studied using an atom–vacancy ordering in titanium monoxide TiO_1.0. The thermodynamic calculations show that the proposed structural modification is equilibrium and must form instead of assumed high-temperature cubic phase (Ti₅O₅)_cub.     

Studies of characteristic flash X-ray lines

Level crossings obeying the selection ruleΔm=2 have been detected in the 6² P _3/2-state of both Rb⁸⁵ and Rb⁸⁷. Using the measured crossing points, the ratios of the hyperfinestructure constants to theg _J -factor are determined from a calculation for intermediate magnetic fields. The ratios areA ₈₅/g _J =6.119 (3) Mc/s,B ₈₅/g _J =6.14 (2) Mc/s,A ₈₇/g _J =20.70 (3) Mc/s,B ₈₇/g _J =2.93 (5) Mc/s. From the zero field level crossing of Rb⁸⁷ the lifetimeτ=1.14 (6) · 10^?7sec of the 6²P_3/2-state is obtained. Level crossing signals have also been observed in the presence of noble gas admixtures.     

Paramagnetische Resonanz von Er3+ in Y2O3

The paramagnetic resonance absorption of trivalent erbium in single crystals of Y₂O₃ on sites of crystal field symmetry C_3i and C₂ is investigated at 4.2°K and 9.2 kMc/s. The values of theg-tensors and those of the hyperfine structure parallel to the axes of crystalline fields are:g _‖=12.176,g _⊥=3.319,A=426.4·10^?4 cm^?1, andg _z =12.314,g _x =1.645,g _y =4.892, andA _z =433.2·10^?4 cm^?1 for the C_3i-ions and the C₂-ions, respectively. For ions on sites of symmetry C₂ the principal axes ofg in the plane perpendicular toz are found ± 2° beside the [100]-directions. This is different from the result on Yb³⁺ in Y₂O₃. The dependency ofg on the angle of rotation is determined for the (001)-, (110)-, and (111)-plane.