Two-phonon scattering of magnetorotons in fractional quantum Hall liquids

We study the phonon-assisted process of dissociation of a magnetoroton, in a fractional quantum Hall liquid, into an unbound pair of quasiparticles. Whilst the dissociation is forbidden to first order in the electron-phonon interaction, it can occur as a two-phonon process. Depending on the value of final separation between the quasiparticles, the dissociation is either a single event involving absorption of one phonon and emission of another phonon of similar energy, or a two-phonon diffusion of a quasiexciton in momentum space. The dependence of the magnetoroton dissociation time on the filling factor of the incompressible liquid is found.Comment: 4 pages, no figure

Volume element structure and roton-maxon-phonon excitations in superfluid helium beyond the Gross-Pitaevskii approximation

We propose a theory which deals with the structure and interactions of volume elements in liquid helium II. The approach consists of two nested models linked via parametric space. The short-wavelength part describes the interior structure of the fluid element using a non-perturbative approach based on the logarithmic wave equation; it suggests the Gaussian-like behaviour of the element's interior density and interparticle interaction potential. The long-wavelength part is the quantum many-body theory of such elements which deals with their dynamics and interactions. Our approach leads to a unified description of the phonon, maxon and roton excitations, and has noteworthy agreement with experiment: with one essential parameter to fit we reproduce at high accuracy not only the roton minimum but also the neighboring local maximum as well as the sound velocity and structure factor.Comment: 9 pages, 6 figure

Attenuation of high frequency phonons in liquid He II

SIGLEAvailable from British Library Document Supply Centre- DSC:DX79438 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Трехфононные взаимодействия и начальный этап эволюции фононного пучка в He II

Исходя из гидродинамического гамильтониана Ландау получено выражение для характерной частоты трехфононных процессов в сверхтекучем ⁴He, справедливое во всем диапазоне фононных энергий, при которых разрешены трехфононные процессы. Проанализированы возможные предельные случаи и проведено сопоставление с результатами предыдущих исследований. Оказалось, что трехфононные процессы полностью определяют начальную релаксацию фононного пучка, инжектированного в He II нагретым твердым телом, так что за время порядка 10⁻¹⁰с в аномальной области фононной дисперсии устанавливается равновесная форма распределения фононов.Виходячи з гідродинамічного гамільтоніану Ландау здобуто вираз для характерної частоти трьохфононних процесів у надплинному ⁴He, який є справедливим у всьому діапазоні фононних енергій, при яких дозволені трьохфононні процеси. Проаналізовано можливі граничні випадки та проведено порівняння з результатами попередніх досліджень. Виявилося, що трьохфононні процеси повністю визначають начальну релаксацію фононного пучка, інжектованого у He II нагрітим твердим тілом, так що за час порядка 10⁻¹⁰с в аномальній області фононної дисперсії встановлюється рівноважна форма розподілу фононів.Starting from the hydrodynamic Landau Hamiltonian we derived an expression for the characteristic rate of three-phonon processes in superfluid ⁴He, which is valid in the whole range of phonon energies where these processes are allowed. Possible limiting cases are analized and a comparison with the results of previous investigations is done. The three-phonon processes are turned out to govern completely the initial relaxation of a phonon pulse injected into He II by a heated solid. This results in an equilibrium phonon distribution in the anomalous region of phonon dispersion at times of the order 10⁻¹⁰s

Experimental Setup for Neutron Lifetime Measurements with a Large Gravitational Trap at Low Temperatures

International audienceCurrently the most accurate measurements of neutron lifetime are being performed at the Petersburg Nuclear Physics Institute with ultracold neutrons (UCNs) stored in a gravitational trap. A modified setup with a large gravitational trap and cooling to 10–15 K is presented. The results of measurements of temperature dependence of UCN losses in collisions with walls, which were coated with a perfluorinated grease (Fomblin UT 18), at 300–77 K, are detailed. The probable heat inflow to the trap is estimated, and the feasibility of cooling to indicated temperatures in experiments with the modified setup is demonstrated