High-energy scattering in a two-dimensional gauge theory

We study high-energy scattering in two-dimensional quantum chromodynamics in the 1/N_colour expansion. To first order, high-energy forward scattering amplitudes are power-behaved with the factorization, signature and exchange degeneracy properties found in dual Regge models. Backward scattering has deviations from naive quark power-counting rules which are related to the form factor and “Regge” behaviours. In the second order of 1/N_colour vacuum exchange diagrams do not give a new Pomeron singularity, but do break exchange degeneracy.     

The phase transition of two-dimensional squaric acid H2C4O4 studied by neutron scattering

At T_c = 370 K squaric acid undergoes a phase transition, which was studied in detail by neutron scattering. The transition is essentially two-dimensional with β = 0.137 ± 0.010. Diffuse scattering above T_e is found to be anisotropic. From “diffuse crystallography” it is concluded that proton disorder is not the dominant source of diffuse scattering. Individual molecules retain their low-temperature shape in the high-temperature phase.     

Raman study of (NH4)2CuCl4 · 2H2O—II: Order-disorder transition

Thc results of a Raman scattering study of the order-disorder phase transition in (NH₄)₂CuCL₄ · 2H₂O are presented. From the variation of intensity of some selected modes in the temperature range 300-100°K, we have established the temperature dependence of the long range order parameter m, defined as the difference between the fractional number of NH₄ ions in the two possible orientations. Since the ammonium network in (NH₄)₂CuCL₄ · 2H₂O is “quasi one-dimensional”, in contrast to the three-dimensional network in NH₄Cl, we discuss the consequence of this feature on the evolution of m.     

Jahn-Teller phase transition in CsCuCl3 and CsCrCl3: A neutron scattering study

The diffuse scattering above T_c was studied in CsCuCl₃ and CsCrCl₃ by neutron scattering technique. These compounds undergo phase transitions associated with the Jahn-Teller effect. Their structures are characterized by isolated chains of face sharing CuCl₆ and CrCl₆ octahedra, respectively. The diffuse scattering shows two main features: it does not occur at the superllatice points of the low temperature phase, but is centred around the strong Bragg points, and its shape changes from one Bragg point to another. This behaviour, which is incompatible with critical scattering from intra-chain correlations of Jahn-Teller distorted octahedra, can be interpreted as “dense impurity” Huang scattering.     

On the Energy of a “One-Dimensional” Two-Electron Atom

Based on the perturbation theory and variational method long known for a “three-dimensional” atom, the ground and first excited state energies are calculated for a “one-dimensional” two-electron atom in the “one-dimensional ortho-helium” configuration, which can be obtained experimentally in principle, as has been already done for a Na Bose condensate, or produced in a super strong magnetic field B ? (2α)²B₀ (B₀ = m²c³/e? ≈ 4.41 × 10¹³ G). The “screening constant” σ for this atom in the ground and excited states was about 0.20 and 0.17, 0.18, respectively, depending on the relative parity PP' of the electronic states, which is somewhat smaller than in “two-dimensional” and “three-dimensional” variants (in these cases, this constant in the ground state is almost the same and about 0.3). The frequencies of the main spectral lines of a “onedimensional” He atom representing a doublet split over the relative parity PP' are found. The presence of the close lines of this doublet in the emission spectrum of magnetars at frequencies ω_{1, 2} ≈ {1.15; 1.17}α²(c/λC) (α = e²/?c, λ_C =?/mc) corresponding to the “one-dimensional ortho-helium” would suggest the existence of a superstrong magnetic field in such astrophysical objects.     

Cyclotron resonance studies on bulk and two-dimensional conduction electrons in InSe

Cyclotron resonance is reported for both bulk electrons and electrons bound to charged defect planes in the layer compound InSe. At temperatures below 20 K all carriers present are bound to defect planes and behave as two-dimensional accumulation layers. At higher temperatures the electrons are excited into bulk regions between the defects, and show three-dimensional, bulk behaviour. The conduction band is shown to exhibit an “anomalous anisotropy” with m_∥ = 0.08m₀ and m_⊥ = 0.14m₀. The bound, two-dimensional carriers exhibit a strong non-parabolicity, and show a band edge mass m_⊥ = 0.13 which is considerably lower than the bulk value possibly due to a reduction in the polaron constant in the degenerate electron gas.     

Hadron scattering in two-dimensional QCD: (I). Formalism and leading order calculations

Two-dimensional quantum chromodynamics in the 1/N_c expansion is explored as a model for hadronic interactions. In meson-meson scattering, order 1/N_c terms exhibit embryonic “Regge” behaviour, with the crossed channel factorization, signature and exchange degeneracy properties found in dual Regge models. “Regge” intercepts are additive in the quark quantum numbers exchanged, and have a lower limit for large quark masses. Residues also factorize in the direct channel. An analogue for fixed angle scattering is presented which has deviations from dimensional counting rules related to form factors and Regge intercepts.     

Finitely separated multimonopoles generated as solitons

A family of 4n?1 parameter solutions corresponding to n monopoles is constructed by a three-dimensional “inverse scattering” method. Global regularity of the finitely separated two-monopole solution is shown.     

Coupled-optical-phonon-mode electron scattering in perovskite oxides

Conductivity and Hall effect measurements were made on KTa_1?xNb_xO₃(x= 0.00,0.04,0.11 and 0.16) in the intermediate temperature range (77–300 K). It was found that the temperatures dependence of the Hall mobility obeyed a semiempirical relationship of Wemple et al. in this temperature range. The main scattering mechanism is proposed to be longitudinal optical phonons which are coupled to the “soft” transverse optical phonons associated with ferroelectricity.     

Magnetoresistance of a Two-Dimensional TbTe<Subscript>3</Subscript> Conductor in the Sliding Charge-Density Wave Regime

The magnetoresistance of a TbTe₃ two-dimensional conductor with a charge-density wave (CDW) has been measured in a wide temperature range and in magnetic fields of up to 17 T. At temperatures well below the Peierls transition temperature and in high magnetic fields, the magnetoresistance exhibits a linear dependence on the magnetic field caused by the scattering of normal charge carriers by “hot” spots of the Fermi surface. In the sliding CDW regime in low magnetic fields, a qualitative change in the magnetoresistance has been observed associated with the strong scattering of carriers by the sliding CDW.     

Specific features of the central peak in the Raman spectra of a lithium niobate crystal

The low-frequency Raman scattering (RS) spectra of a LiNbO₃ ferroelectric crystal are studied in the temperature range 300–1423 K. The central peak characterizing the relaxation susceptibility of the crystal lattice is observed over the entire temperature range studied, including at temperatures much lower than the Curie temperature (T _c = 1470 K). Far from T _c, the characteristics of the central peak are shown to be unlike those expected in the framework of the standard approaches. (i) The central-peak width γ_R increases as the temperature increases to T ～ 1300 K, and the critical slowing down (γ_R ∝ T _c ? T) occurs only above this temperature. (ii) A central peak arises in the RS geometry where the scattering by “nonferroelectric” E phonons is allowed. The experimental results are interpreted with allowance for relaxation dynamics in local regions of the crystal lattice.     

Thermoelectric power of magnetic metals: II. Anisotropic metals

We present a theory of the thermoelectric power tensor of anisotropic ferromagnetic metals with localized magnetic moments starting from the Boltzmann equation and incorporating anisotropy effects due to the lattice structure through a parameter measuring the anisotropy in the sound velocity. Elastic and inelastic phonon and spin scattering contributions are taken into account through a linear superposition of scattering cross sections. A mean field approximation is used to describe the ordered magnetic phase. Spin wave and impurity scattering, phonon and magnon drag are not included. In a range encompassing the Curie temperature, i.e. at “moderate temperatures”, the theory quantitatively reproduces observed features except for specific details (e.g. rounding near T_c) needing other physical input. We compare our theory to data on single crystals of Gd and Tb₇₅Gd₂₅. The c-axis thermoelectric power is well recovered for very reasonable values of anisotropy and scattering strength parameters. A conjecture is given to explain the basal plane thermoelectric power positive slope at high temperature.     

Transparent potentials at fixed energy in dimension two. Fixed-energy dispersion relations for the fast decaying potentials

For the two-dimensional Schrödinger equation $$[ - \Delta + v(x)]\psi = E\psi , x \in \mathbb{R}^2 , E = E_{fixed} > 0 (*)$$ at a fixed positive energy with a fast decaying at infinity potentialv(x) dispersion relations on the scattering data are given. Under “small norm” assumption using these dispersion relations we give (without a complete proof of sufficiency) a characterization of scattering data for the potentials from the Schwartz classS=C _∞ ^(∞) (?²). For the potentials with zero scattering amplitude at a fixed energyE _fixed (transparent potentials) we give a complete proof of this characterization. As a consequence we construct a family (parametrized by a function of one variable) of two-dimensional spherically-symmetric real potentials from the Schwartz classS transparent at a given energy. For the two-dimensional case (without assumption that the potential is small) we show that there are no nonzero real exponentially decreasing, at infinity, potentials transparent at a fixed energy. For any dimension greater or equal to 1 we prove that there are no nonzero real potentials with zero forward scattering amplitude at an energy interval. We show that KdV-type equations in dimension 2+1 related with the scattering problem (*) (the Novikov-Veselov equations) do not preserve, in general, these dispersion relations starting from the second one. As a corollary these equations do not preserve, in general, the decay rate faster than |x|^?3 for initial data from the Schwartz class.     

“Thermal barrier” effect in energy relaxation of polaritons in HgI2

The paper presents the results of measurements of polariton radiation spectra, polariton band excitation spectra and spectra of resonance Raman scattering HgI₂. These are registered at different temperatures and units of energetic derangement between the exciting radiation quantum energy and that of transverse exciton. The threshold value of the temperature (T=12 K) starting from which there is attained the equilibrium between the polariton and phonon subsystems is found. The scattering mechanism that guides the formation of “thermal barrier” favouring the polaritons thermalization, is revealed.     

A topological approach to high-energy large transverse momentum interactions

We discuss the high p_T strong interaction from a topological (1/N_f) expansion viewpoint. Topology of quark line graphs is used to classify the underlying process rather than constituent scattering, all constituents being implicitly confined. The distinction between “jets” and “background” is clarified in a very simple dynamical model. Although the model does not require quarks as dynamical entities, many of its features are in agreement with parton model predictions.     

Inelastic neutron scattering study of commensurate-incommensurate phase transition in thiourea

This paper presents a study of the nature of the incommensurate lattice instability in deuterated thiourea by inelastic neutron scattering. It shows clearly a “soft mode” which condenses at the phase transition in a satellite reflexion. A study of related dispersion curves along the [O ξ O] direction shows an unusual spectral shape in a large temperature range above T_c.     

A study of Fe-doped Bi-Sr-Ca-Cu superconductors

We have substituted 1.5% of Fe for Cu in several “2212” and “2223” Bi-Sr-Ca-Cu superconductors. All of the samples show a reduction ofT _c about 13 K due to the Fe impurities. Mössbauer measurements at room temperature reveal the structural characteristics such as stacking faults and intergrowth of different phases in these Bi-based compounds on the microscopic scale. The susceptibility ofT _c to Fe-doping in the Bi-“2212” or “2223” system is comparable to that of the “123” system but much smaller than that of the “214” system. The interplanar correlation existing in the “123” and the Bi-“2212” and “2223” systems seems to play an important role in sustaining the high temperature superconductivity and weakening the detrimental effect of impurity elements on superconductivity in these two systems.     

Energy relaxation of electrons in the (100) n-channel of a Si-MOSFET: II. Surface phonon treatment

The electron energy relaxation is investigated as a function of the “electron temperature” T_e in the n-channel of a (100) surface silicon MOSFET device by inspecting the phenomenological energy relaxation time τ_ε(T_e). τ_ε is determined theoretically and compared to experimental results in order to identify the energy relaxation mechanism(s) present at the interface. Two dimensional electron transport is assumed. Single activation temperature (θ) Rayleigh wave scattering and acoustic Rayleigh wave scattering are studied as possible energy loss processes. The effects of electric subbanding near the surface are included. τ_ε is calculated for T_e ? 15 K in the electric quantum limit. We find that a single θ = 12.0 K Rayleigh phonon fits theory to experiment for a single electron inversion density (N_inv) case, but can not provide a fit simultaneously for more than one N_inv value. Theory and experiment disagree when Rayleigh wave acoustic scattering is assumed.