Pressure Effects on Spectra of Tunable Laser Crystal GSGG:Cr3+ Ⅳ:Pressure-Induced Shifts of R1 Line, R2 Line, and U Band at 300 K

By means of both a theory for pressure-induced shifts (PS) of energy spectra and a theory for shifts of R2 line, and U band of GSGG:Cr3+ at 300 K have been calculated, respectively. The calculated results are in good agreement with all the experimental data. Their physical origins have also been explained. It is found that the mixingdegree of \t22(3T1)e4T2> and \t322E> base-wavefunctions in the wavefunctions of R1 level of GSGG:Cr3+ at 300 K is remarkable under normal pressure, and the mixing-degree rapidly decreases with increasing pressure. The change of the and the PS of R1 line (or R2 line) due to EPI are quite different. It is the combined effect of them that gives rise to the total PS ofR1 line (or R2 line). In the range of about 15 kbar ～ 45 kbar, the mergence and/or order-reversal between t22(3T1)e4T2 levels and t32 2 T1 levels take place, which cause the fluctuation of the rate of PS for t22(3T1)e4T2 (or t3 2 2T1)with pressure. At 300 K, both the temperature-dependent contribution to R1 line (or R2 line or U band) from EPI and the temperature-independent one are important.     

Weak localization and fluctuation electron-electron interaction in disordered indium films

The effects of perpendicular magnetic fieldsH on the temperature dependence of the resistanceR(T) (T<20 K), the superconducting transition temperatureT _c and the fluctuation superconductivity (T«T _c ) have been investigated on high-resistance (R _>1 kOhm) indium films prepared by simultaneous condensation of the metal and hydrogen onto a substrate cooled to 5 K. AtT«T _c the resistance slightly decreased with rising temperature, in agreement with the weak localization and electron-electron interaction (EEI) theories. Processing of theR(T, H) curves in terms of the theories permitted the electron phase relaxation time (T) to be determined and the time _so of electron spin relaxation due to spin-orbital interaction (SOI) to be estimated. In the temperature rangeT–T _c /k the effect of the fluctuation EEI on conductivity was investigated. The contribution of the fluctuation EEI is shown to be given by a sum of two corrections: the Cooper and the Maki-Thompson ones. In the rangeT–T _c /k the influence of magnetic field was investigated on the transition from two-to zerodimensional fluctuations of the order parameter resulting from temperature increase in the rangeT>T _c . Increasing field was found to cause enlargement of the zero-dimensional fluctuation critical region and lowering of the fluctuation dimensionality alteration temperature, leading eventually to complete two-dimensional fluctuation suppression. The experimental dependencesT _c (H) experienced, in addition to orbital effects of the magnetic field, depairing effect of the field on electron spins. Thus the _so magnitude could also be estimated and proved roughly equal to _so determined from the analysis ofR(T, H)     

Pressure-Induced Shifts of R1 and R2 Lines of YAG:Cr3+

By means of improved ligand-field theory, the "pure electronic" pressure-induced shifts (PS's) and the PS's due to electron-phonon interaction (EPI) of R1 line and R2 line of YAG:Cr3 have been calculated, respectively.The calculated results are in very good agreement with the experimental data. It is demonstrated that the admixture of |t22(3T1)e4T2> and |t322E> bases in the wavefunction of R1 level of YAG:Cr3 and its change with pressure play a key role for the PS of R1 line. The behaviors of the "pure electronic" PS of R1 line and the PS of R1 line due to EPI are different. It is the combined effect of them that gives rise to the total PS of R1 line, which has satisfactorily explained the experimental results. The systematic analyses and comparisons between the feature of R1-line PS of YAG:Cr3 and the ones of three laser crystals (GSGG:Cr3 , GGG:Cr3 and ruby) have been made, and the origin of the difference between them has been revealed.     

Interacting electrons in disordered wires: Anderson localization and low-T transport

We study the conductivity sigma(T) of interacting electrons in a low-dimensional disordered system at low temperature T. For weak interactions, the weak-localization regime crosses over with lowering T into a dephasing-induced "power-law hopping." As T is further decreased, the Anderson localization in Fock space crucially affects sigma(T), inducing a transition at T = T(c), so that sigma(T < T(c)) = 0. The critical behavior of sigma(T) above T(c) is ln sigma(T) proportional to -(T - T(c))(-1/2). The mechanism of transport in the critical regime is many-particle transitions between distant states in Fock space.     

Energy distribution and energy fluctuation in Tsallis statistics

The energy distribution and the energy fluctuation in the Tsallis canonical ensemble are studied with the OLM formalism but following a new way. The resulting formula for the energy fluctuation is not the same as that in previous work [L.Y. Liu, J.L. Du, Physica A 387 (2008) 5417]. In discussing the application of an ideal gas, we find that the energy fluctuation can not be negligible in the thermodynamic limit, showing the ensemble nonequivalence for this case in Tsallis statistics. We investigate the energy fluctuation with a Tsallis generalized canonical distribution studied by Plastino and Plastino [A.R. Plastino, A. Plastino, Phys. Lett. A 193 (1994) 140] for describing a system in contact with a finite heat bath. For this situation, the two formulae for the energy fluctuation are shown to be equivalent, while the nonextensive parameter q$q$ plays a very important role.     

Multichannel time-reversal processing for acoustic communications in a highly reverberant environment

The development of time-reversal (T/R) communication systems is a recent signal processing research area dominated by applying T/R techniques to communicate in hostile environments. The fundamental concept is based on time-reversing the impulse response or Green's function characterizing the uncertain communications channel to mitigate deleterious dispersion and multipath effects. In this paper, we extend point-to-point to array-to-point communications by first establishing the basic theory to define and solve the underlying multichannel communications problem and then developing various realizations of the resulting T/R receivers. We show that not only do these receivers perform well in a hostile environment, but they also can be implemented with a "1 bit" analog-to-digital converter design structure. We validate these results by performing proof-of-principle acoustic communications experiments in air. It is shown that the resulting T/R receivers are capable of extracting the transmitted coded sequence from noisy microphone array measurements with zero-bit error.     

Breaking of Cubic Symmetry in Rare-Earth Dodecaborides with Dynamic Charge Stripes

JETP Letters - The effect of symmetry lowering for the parameters of electron transport in rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Lu) with fcc crystal structure at temperatures T ≤...     

Braidings of Tensor Spaces

Let V be a braided vector space, i.e., a vector space together with a solution \({\hat{R}\in {{End}}(V\otimes V)}\) of the Yang–Baxter equation. Denote \({T(V):=\bigoplus_k V^{\otimes k}}\) . We associate to \({\hat{R}}\) a one-parameter family of solutions \({T(\hat{R})\in {\rm End}(T(V)\otimes T(V))}\) of the Yang–Baxter equation on the tensor space T (V). Main ingredients of the solution are braid analogues of the binomial coefficients and of the Pochhammer symbols. The association \({\hat{R}\rightsquigarrow T(\hat{R})}\) is functorial with respect to V.     

Nature of the transition between a ferromagnetic metal and a spin-glass insulator in pyrochlore molybdates

The metal-insulator transition has been investigated for pyrochlore molybdates R(2)Mo(2)O(7) with nonmagnetic rare-earth ions R. The dynamical scaling analysis of ac susceptibility reveals that the geometrical frustration causes the atomic spin-glass state. The reentrant spin-glass phase exists below the ferromagnetic transition. The electronic specific heat is enhanced as compared to the band calculation result, perhaps due to the orbital fluctuation in the half-metallic ferromagnetic state. The large specific heat is rather reduced upon the transition, likely because the short-range antiferromagnetic fluctuation shrinks the Fermi surface.     

Ion population fraction calculations using improved screened hydrogenic model with l-splitting

Ion population fraction(IPF) calculations are very important to understand the radiative spectrum emitted from the hot dense matter. IPF calculations require detailed knowledge of all the ions and correlation interactions between the electrons of an ion which are present in a plasma environment. The average atom models, e.g., screened hydrogenic model with l-splitting(SHML), now have the capabilities for such calculations and are becoming more popular for in line plasma calculations. In our previous work [Ali A, Shabbir Naz G, Shahzad M S, Kouser R, Rehman A and Nasim M H 2018 High Energy Density Phys. 26 48], we have improved the continuum lowering model and included the exchange and correlation effects in SHML. This study presents the calculation of IPF using classical theory of fluctuation for our improved screened hydrogenic model with l-splitting(I-SHML) under local thermodynamic equilibrium conditions for iron and aluminum plasma over a wide range of densities and temperatures. We have compared our results with other models and have found a very good agreement among them.     

Generalized entropy production fluctuation theorems for quantum systems

Based on trajectory-dependent path probability formalism in state space, we derive generalized entropy production fluctuation relations for a quantum system in the presence of measurement and feedback. We have obtained these results for three different cases: (i) the system is evolving in isolation from its surroundings; (ii) the system being weakly coupled to a heat bath; and (iii) system in contact with reservoir using quantum Crooks fluctuation theorem. In Case (iii), we build on the treatment carried out by H T Quan and H Dong [arXiv/cond-mat:0812.4955], where a quantum trajectory has been defined as a sequence of alternating work and heat steps. The obtained entropy production fluctuation theorems (FTs) retain the same form as in the classical case. The inequality of second law of thermodynamics gets modified in the presence of information. These FTs are robust against intermediate measurements of any observable performed with respect to von Neumann projective measurements as well as weak or positive operator-valued measurements.     

Magnetic ordering in double perovskites R2CoMnO6 (R = Y, Tb) investigated by high resolution neutron spectroscopy

We have investigated low energy nuclear spin excitations in double perovskite compounds R(2)CoMnO(6) (R=Y, Tb) by inelastic neutron scattering with a high resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 μeV for Y(2)CoMnO(6) and also for Tb(2)CoMnO(6) at T = 2 K in both energy-loss and energy-gain sides. We interpret these inelastic peaks to be due to the transitions between the hyperfine split nuclear levels of the (59)Co nucleus. The inelastic peaks move towards the central elastic peak and finally merge with it at the magnetic ordering temperature T(C). The energy of the low energy excitations decreases continuously and becomes zero at T(C) ≈ 75 K for Y(2)CoMnO(6) and T(C) ≈ 100 K for Tb(2)CoMnO(6). For Tb(2)CoMnO(6), which contains magnetic rare earth ions, additional quasielastic scattering due presumably to the fluctuations of large Tb magnetic moments was observed. The present study reveals the magnetic ordering of the Co sublattice. The results of this investigation along with that obtained by us for other compounds indicate the presence of unquenched orbital moments in some of the Co compounds.     

THE BUTATRIENE-TO-ACETYLENE TRANSITION IN POLYDIACETYLENE DUE TO THERMAL AND QUANTUM FLUCTUATION

The butatriene-to-acetylene transition in polydiacetylene(PD) due to thermal and quantum fluctuation is investigated by using a simple continuum model.We started from general principle of statistical mechanics to discuss the rate of thermal and quantum nucleation. It is concluded that the structure-phase-transition is due to thermal fluctuation at higher, and quantum fluctuation at lower temperature mainly.It agrees with the experimental results.     

Phase coexistence between the (√3 × √3)R30° - β and (1 × 1) phases on Pb/Ge(1 1 1)

We have characterized the phase transition between the (1 × 1) and (√3 × √3)R30° - β phases on Pb/Ge(1 1 1) using low energy electron microscopy (LEEM). We show that the transition is first-order and that, in the coexistence region of the two phases, the dominant mechanism for phase separation changes critically with Pb coverage, from nucleation and growth at 1.33 ML (saturation coverage of the β phase) to spontaneous domain switching due to thermal fluctuations of the local Pb density for slightly smaller coverage. As the Pb coverage decreases, the concentration of vacancies in the β phase increases, making additional possible Pb adsorption sites available. The larger resulting local density fluctuation of Pb becomes comparable to the density difference of the two phases, manifesting itself in the observed domain switching.     

Antiferromagnetic spin fluctuations above the dome-shaped and full-gap superconducting states of LaFeAsO1-xFx revealed by (75)As-nuclear quadrupole resonance

We report a systematic study by (75)As nuclear-quadrupole resonance in LaFeAsO(1-x)F(x). The antiferromagnetic spin fluctuation found above the magnetic ordering temperature T(N) = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T(c) is found, with the highest T(c) = 27 K at x = 0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T(c). These features resemble closely the cuprates La(2-x)Sr(x)CuO(4). In x = 0.06, the spin-lattice relaxation rate (1/T(1)) below T(c) decreases exponentially down to 0.13T(c), which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T(1) below T(c) is rendered nonexponential for other x by impurity scattering.     

Nernst effect and disorder in the normal state of high-T(c) cuprates

We have studied the influence of disorder induced by electron irradiation on the Nernst effect in optimally and underdoped YBa2Cu3O(7-delta) single crystals. The fluctuation regime above T(c) expands significantly with disorder, indicating that the T(c) decrease is partly due to the induced loss of phase coherence. In pure crystals the temperature extension of the Nernst signal is found to be narrow whatever the hole doping, contrary to data reported in the low-T(c) cuprate families. Our results show that the presence of intrinsic disorder can explain the enhanced range of the Nernst signal found in the pseudogap phase of the latter compounds.     

Multiple-Scattering Contributions to the Electrical Conductivity of Non-Ideal Plasmas. I. Two Particle Scattering

On the basis of the Green's function formalism multiple scattering effects due to two-particle scattering are investigated with respect to their influence on the electrical conductivity of dense, non-ideal hydrogen plasmas. Both the linear response formalism and the rigorous kinetic treatment via the Kadanoff-Baym equation yield equivalent results for this case. A local lowering of the conductance isotherms for is found to be a consequence of the disappearence of the last bound state of the shielded electron-proton complex. Influences due to lowest order dynamical shielding effects are found to cause considerable lowering of the conductivity in this region due to photon polarization. We also discuss effects of multiple two-particle scattering on the photon-polarization itself.     

The symmetry properties of nearest neighbour Cu2+ ions in a square planar lattice

The symmetry of the antiferromagnetic Heisenberg exchange interaction is lowered by dipolartype interactions. It is reasoned that the combination of small transition matrix elements and the zero field splittings due to the symmetry lowering can account for the absence of E.S.R. signals.My thanks are due to the Leverhulme Trust for an Emeritus Fellowship during the tenure of which this investigation was carried out.     

CW Operation of a Submillimeter Wave Gyrotron (Gyrotron Fu IV) for High Stability of the Output Frequency

The first cw operation of our submillimeter wave gyrotron (Gyrotron FU IV) using a 12 T superconducting magnet has been successfully carried out. Output power is more than 20 W at a frequency of 301 GHz in the TE₀₃₁ resonant cavity mode. Time-resolved frequency measurement s shows that the frequency fluctuation of the gyrotron output is smaller than 2 MHz. This frequency fluctuation is mainly due to the fluctuation in the output voltage of the power supply.     

Intermittency in drift-wave turbulence: structure of the momentum flux probability distribution function

We analytically compute the probability distribution function (PDF) of the local Reynolds stress ( R) for forced Hasegawa-Mima turbulence. With the assumption that the PDF tail is due to an instanton with the spatial form given by the modon solution, the tail of the PDF of R is found to be a stretched, non-Gaussian exponential, with the specific form exp[-cR(3/2)] ( c is a constant). We relate the temporal localization of the instanton to the degree of "burstiness" of the momentum transport event.