Superfluidity

With such a general model, one cannot obtain numerical results, but one can obtain general results which appear to correspond to some of the peculiar properties of liquid helium. Eq. (1) is not the exact Hamiltonian; it can be derived from the Hamil- tonian of a system of ...

From measurements of the magnetic penetration depth, $\lambda(T)$, from 1.6 K to $T_c$ in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, $N_s(E)$ at T=0 by using a simple model. In this framework, the flat behavior of $\lambda^{-2}(T)$ at low $T$ implies $N_s(E)$ is small, possibly gapped, at low energies. The upward curvature in $\lambda^{-2}(T)$ near $T_c$ seen in overdoped films implies that superfluid comes from an anomalously small energy band within about $3k_BT_c$ of the Fermi surface.

The hydrodynamic description of a superfluid is usually based on a two-fluid picture. In this thesis, basic properties of such a relativistic two-fluid system are derived from the underlying microscopic physics of a complex scalar quantum field theory. To obtain analytic results of all non-dissipative hydrodynamic quantities in terms of field theoretic variables, calculations are first carried out in a lowtemperature and weak-coupling approximation. In a second step, the 2-particle-irreducible formalism is applied: This formalism allows for a numerical evaluation of the hydrodynamic parameters for all temperatures below the critical temperature. In addition, a system of two coupled superfluids is studied. As an application, the velocities of first and second sound in the presence of a superflow are calculated. The results show that first (second) sound evolves from a density (temperature) wave at low temperatures to a temperature (density) wave at high temperatures. This role reversal is investigated for ultra-relativistic and near-nonrelativistic systems for zero and nonzero superflow. The studies carried out in this thesis are of a very general nature as one does not have to specify the
system for which the microscopic field theory is an effective description. As a particular example, superfluidity in dense quark and nuclear matter in compact stars are discussed.

This thesis presents an experimental study of hydrodynamical phenomena of a laser propagating nonlinearly. For a medium presenting an intensity-dependent refractive index, and in the frame of the paraxial approximation, the intensity of the laser beam is equivalent to a density of a fluid, the propagation direction is seen as a time evolution of the fluid as well as the phase gradient of the laser beam defines a flow velocity and the nonlinear refractive index change allows defining a sound velocity of the fluid. Under this analogy, we call the propagating laser beam a fluid of light. In this thesis, we provide a study of the superfluidity concept of a fluid of light in a self-defocusing regime of the nonlinearity. It is defined as the absence of diffraction when the fluid of light encounters an obstacle. The parameters which control the superfluid transition are: the flow velocity as well as the sound velocity. They are monitored respectively through the wave vector and the intensity of the laser beam. In the frame of this analogy, we also present in this manuscript a study of vortex shedding regime as a result of the interaction between the fluid of light and the obstacle. Here, the obstacle is considered being strong. When twice the flow velocity at the poles of the obstacle is larger than the sound velocity, pairs of vortex/anti-vortex are emitted demonstrating a hydrodynamical behaviour of the fluid of light. In order to underline the nonlinear refractive index change, we also report in this thesis a study of the photorefractive nonlinearity using the self-phase modulation effect.

Il substrato universale, la fabbrica dello spaziotempo, contiene correnti ben più complesse delle onde gravitazionali trasversali teorizzate da Einstein. Qui offriamo alcune considerazioni su queste dinamiche longitudinali alla luce delle rilevazioni di LIGO del 2016.

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Researchers at Dartmouth College have built the world's first superfluid circuit that uses pairs of ultracold electron-like atoms, according to a study published in Physical Review Letters.
Physicists at the University of Queensland have shed light upon how tiny whirlpools (vortices) get stuck to obstacles in superfluids. [27]
We are currently designing a new mechanical oscillator that can probe the structural order at this stage." [26]
Researchers at Rochester Institute of Technology are part of a new study that could help unlock the potential of superfluids—essentially frictionless special substances capable of unstopped motion once initiated. [25]

This paper explores an alternative to the metaphysically barren traditional view of electromagnetism we have inherited, by hydrodynamically explaining magnetic and electric fields in terms of vacuum fluid flow. Once the vacuum is taken to be a real physical medium composed of interacting and dynamically fluid quanta (like a superfluid), electromagnetism can be naturally derived from well-known principles. Through this lens, a vector field, representing the positions and velocities of all quanta within the vacuum fluid, becomes ontologically prior to electromagnetism and the Helmholtz theorem can be used to generally characterize the fluid flow of that flux vector field. Decomposing that characterization into independent expressions for curl and divergence automatically maps out the magnetic and electric fields; providing a physical model for electromagnetism in which magnetic flux is interpreted as the curl of the quantum vector field and the electric field is interpreted as divergence in that vector field.

В работе на основе теории строения керна электрона рассмотрен механизм взаимодействия двух и более электронов. Отмечается, что электроны могут находиться только в двух устойчивых состояниях: с параллельным или антипараллельным спином. Рассмотрены различные случаи взаимного положения электронов. Основное внимание уделено осевому положению электронов с антипараллельными спинами на близких расстояниях, когда керны электронов частично перекрываются. Показано, что в этом случае образуются электронные пары, а керны электронов значительно увеличивают свои размеры. Электронные пары могут объединяться в более сложные структуры, электронные цепи, образующие электронные оболочки атомов. Объединяя свои внешние оболочки в более длинную цепь, два атома могут связаться в молекулу. Свободные электроны в металлах также могут объединяться в куперовские электронные пары. В свою очередь, при достаточном охлаждении куперовские пары образуют электронные цепи, поверхностный слой которых обладает свойством сверхпроводимости.

We present proof-of-concept measurements of the vortex line density generated by a quartz tuning fork resonator probed by the attenuation of second sound in superfluid 4He at 1.6 K. The force–velocity response of a quartz tuning fork operating at a frequency of 31 kHz exhibited the onset of extra damping at a velocity of 0.5 ms −1. Attenuation of the 5th resonant mode of second sound was observed at the same velocity , indicating the production of vortex lines. Our measurements demonstrate that an increase of the drag coefficient corresponds to the development of quantum turbulence.

It is well known that the hydrodynamics of a zero-temperature superfluid can be formulated in
field-theoretic terms, relating for example the superfluid four-velocity to the gradient of the phase
of a Bose-condensed scalar field. At nonzero temperatures, where the phenomenology of a superfluid
is usually described within a two-fluid picture, this relationship is less obvious. For the
case of a uniform, dissipationless superfluid at small temperatures and weak coupling we discuss
this relationship within a j4 model. For instance, we compute the entrainment coefficient, which
describes the interaction between the superfluid and the normal-fluid components, and the velocities
of first and second sound in the presence of a superflow. Our study is very general, but can
also be seen as a step towards understanding the superfluid properties of various phases of dense
nuclear and quark matter in the interior of compact stars.

Interaction mechanism between two or more electrons is considered in present paper on the basis of theory of electron core structure. It is pointed out that electrons may be only in two steady states: with parallel or opposite spins. Various cases are considered differed by mutual position of electrons. Basic attention is given to the axially arranged electrons with opposite spins at short distances, when electron cores are partially overlapped. It is shown that in this case electron pairs are generated and the electrons cores are considerably increased in their sizes. Electron pairs can be merged to form more complicated structures or the electron chains which form electron shells of atoms. Two atoms can be bonded to form a molecule by merging their outer shells what forms longer chain. Free electrons in metals can also merge to generate Cooper pairs. In turn, in sufficient cooling, Cooper pairs generate the electronic chains which surface layer is found to be superconductive.

The hydrodynamic description of a superfluid is usually based on a two-fluid picture. We compute the basic properties of the relativistic two-fluid system from the underlying microscopic physics of a relativistic φ4 complex scalar field theory. We work at nonzero but small temperature and weak coupling, and we neglect dissipation. We clarify the relationship between different formulations of the two-fluid model and how they are parametrized in terms of partly redundant current and momentum four-vectors. As an application, we compute the velocities of first and second sound at small temperatures and in the presence of a superflow. While our results are of a very general nature, we also comment on their interpretation as a step towards the hydrodynamics of the color-flavor locked state of quark matter, which, particularly in the presence of kaon condensation, appears to be a complicated multicomponent fluid.

We consider a family of tight-binding models based on kagome lattice with local fluxes, which have a lowest flat band in the single particle spectrum. The flat band is spanned by eigenstates forming localized loops on the lattice, with the maximally compact loop states typically breaking the discrete rotational symmetry of the lattice.  When populated by locally-interacting particles, the close packing of such maximally compact loop states leads to a nematic Wigner crystal ground state. If particles are bosons, increasing filling beyond the close packing filling fraction, at the mean field level, leads to formation of a nematic supersolid and a nematic superfluid phases with broken lattice rotation and $U(1)$ symmetry.

We reexpress the superfluid density of a disordered superconductor obtained by two of us earlier [Phys. Rev. B 102, 024514 (2020)] in a new highly convergent form, and use the results to make an extensive and successful comparison with experiment in the dirty limit for all temperatures. We point out that there is a regime (conventional superconductor with low, but increasing disorder) where theoretical predictions need to be confronted with accurate experiment.

Bose–Einstein condensates confined in traps exhibit unique features which have been the object of extensive experimental and theoretical studies in the last few years. In this paper I will discuss some issues concerning the behaviour of the order parameter and the dynamic and ...

We consider hardcore bosons in two coupled chain of one dimensional lattices at half filling with repulsive intra-chain interaction and inter-chain attraction. This can be mapped on to a coupled chain of spin-1/2 XXZ model with inter chain ferromagnetic coupling. We investigate various phases of hardcore bosons (and related spin model) at zero temperature by density matrix renormalization group method. Apart from the usual superfluid and density wave phases, pairing of inter chain bosons leads to the formation of novel phases like pair-superfluid and density wave of strongly bound pairs. We discuss the possible experimental realization of such correlated phases in the context of cold dipolar gas.

Evidence for helium excimers (He2(*)) in the lowest allowed rotational quantum state in liquid helium is presented. He2(*) was generated by a corona discharge in the gas and normal liquid phases. Fluorescence spectra recorded in the visible region between 3.8 and 5.0 K and 0.2 and 5.6 bar showed the rotationally resolved d(3)Σu(+) → b(3)Πg transition of He2(*). Analysis of the pressure and temperature dependence of lineshifts and line intensities showed features of solvated He2(*) superimposed on its gas-phase spectrum and, in the liquid phase only, pressure-induced rotational cooling. These findings suggest that He2(*) can be used to investigate bulk helium in different phases at the nanoscale.