Motivated by a recent experiment [S. Beattie, S. Moulder, R. J. Fletcher, and Z. Hadzibabic, PRL 110, 025301 (2013)] we study the superflow of atomic spinor Bose-Einstein condensates optically trapped in a ring-shaped geometry. Within a dissipative mean-field approach we simulate a twocomponent condensate in conditions adapted to the experiment. In qualitative agreement with the experimental findings, we observe persistent currents, if the spin-population imbalance is above some well-defined 'critical' value. The triply charged vortices decay in quantized steps. The vortex lines escape from the center of the ring through dynamically created regions in the condensate annulus with reduced density of one component filled by atoms of the other component. The vortices then leave the ring-shaped high density region of the condensate and finally decay into elementary excitations. Persistent currents or 'flows without friction' as a hallmark of superfluidity have been studied in liquid helium for several decades. Recently, persistent flow of atoms has been observed in Bose-Einstein condensates (BEC) trapped in a ring-like potential [1][2][3][4][5]. This enables fundamental studies of superfluidity and may lead to applications in high precision metrology and atomtronics. Of course, the question of the stability of the atomic persistent currents is of fundamental importance and, therefore, the subject of numerous investigations.Theoretical studies of persistent currents in atomic BEC are mostly limited to the simplified cases of one-dimensionality and very weak interactions. Onecomponent BECs in a one-dimensional (1D) ring potential were studied in Refs. [6] and [7]. Superfluidity on a 1D ring in the presence of impurities was investigated in Ref. [8]. Families of 2D solitary waves with and without singly-charged persistent flow are investigated in Ref. [9]. For two-component BECs in 1D or 2D traps, the stability of the persistent currents and their decay mechanisms were under investigation in Refs. [10][11][12][13][14][15][16]. Smyrnakis et al. [10] concluded that in a strictly one-dimensional ring persistent currents with circulation lager than one are stable only in single-component gases. This conclusion was challenged recently [17]. In Ref.[18] it was found on the basis of mean field calculations supported by exact diagonalization results, that persistent currents in 2D traps may be stable under specific conditions. Experimentally, this problem has been addressed very recently [5] for a toroidally trapped gas of 87 Rb atoms in two different spin states. However, previous theoretical investigations describe the stability of the persistent currents in spinor BEC only qualitatively, but do not elucidate the microscopic mechanism of the instabilities and its impact on the dynamics of the persistent currents.In the present work, we investigate the stability of the superflow within a two-dimensional dissipative meanfield theory. We find that phase-slips occur as the result of the simultaneous action of two factors: an azim...
Motivated by the recent experiment [Wright et al., Phys. Rev. A 88, 063633 (2013)], we investigate formation of vortices in an annular BEC stirred by a narrow blue-detuned optical beam. In the framework of a two-dimensional mean field model, we study the dissipative dynamics of the condensate with parameters matched to the experimental conditions. Vortex-antivortex pairs appear near the center of the stirrer in the bulk of the condensate for slow motion of the stirring beam. When the barrier angular velocity is above some critical value, an outer edge surface mode develops and breaks into the vortices entering the condensate annulus. We determine the conditions for creation of the vortex excitations in the stirred toroidal condensate and compare our results with the experimental observations.Comment: 7 pages 5 figure
Motivated by recent experiments we study the influence of thermal noise on the phase slips in toroidal Bose-Einstein condensates with a rotating weak link. We derive a generalized Arrheniuslike expression for the rate of stochastic phase slips. We develop a method to estimate the energy barrier separating different superflow states. The parameters at which the energy barrier disappears agree with the critical parameters for deterministic phase slips obtained from dynamics simulations, which confirms the validity of our energetic analysis. We reveal that adding thermal noise lowers the phase-slip threshold. However, the quantitative impact of the stochastic phase slips turns out to be too small to explain the significant discrepancy between theoretical and the experimental results.
We present a brief overview of crucial historical stages in creation of superfluidity theory and of the current state of the microscopic theory of superfluid 4He. We pay special attention to the role of Bose-Einstein condensates (BECs) in understanding of physical mechanisms of superfluidity and identification of quantum mechanical structure of 4He superfluid component below λ-point, in particular—the possibility that at least two types of condensates may appear and coexist simultaneously in superfluid 4He. In this context we discuss the properties of the binary mixtures of BECs and types of excitations, which may appear due to intercomponent interaction in such binary mixtures of condensates. We also discuss current status of investigations of persistent currents in toroidal optical traps and present an outlook of our recent findings on this subject.
Purpose: Numerous MRI applications require data from external devices. Such devices are often independent of the MRI system, so synchronizing these data with the MRI data is often tedious and limited to offline use. In this work, a hardware and software system is proposed for acquiring data from external devices during MR imaging, for use online (in real-time) or offline. Methods:The hardware includes a set of external devices -electrocardiography (ECG) devices, respiration sensors, microphone, electronics of the MR system etc. -using various channels for data transmission (analog, digital, optical fibers), all connected to a server through a universal serial bus (USB) hub. The software is based on a flexible client-server architecture, allowing real-time processing pipelines to be configured and executed. Communication protocols and data formats are proposed, in particular for transferring the external device data to an open-source reconstruction software (Gadgetron), for online image reconstruction using external physiological data. The system performance is evaluated in terms of accuracy of the recorded signals and delays involved in the real-time processing tasks. Its flexibility is shown with various applications. Results:The real-time system had low delays and jitters (on the order of 1 ms). Example MRI applications using external devices included: prospectively gated cardiac cine imaging, multi-modal acquisition of the vocal tract (image, sound, and respiration) and online image reconstruction with nonrigid motion correction. Conclusion:The performance of the system and its versatile architecture make it suitable for a wide range of MRI applications requiring online or offline use of external device data. K E Y W O R D Shardware, physiological data, real-time, signal processing, software Karyna Isaieva and Marc Fauvel contributed equally to this work.
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