Collective modes and superfluidity of a two-dimensional ultracold Bose gas

The collective modes of a quantum liquid shape and impact its properties profoundly, including its emergent phenomena such as superfluidity. Here we present how a two-dimensional Bose gas responds to a moving lattice potential. In particular we discuss how the induced heating rate depends on the interaction strength and the temperature. This study is motivated by the recent measurements of Sobirey {\it et al.} arXiv:2005.07607 (2020), for which we provide a quantitative understanding. Going beyond the existing measurements, we demonstrate that this probing method allows to identify first and second sound in quantum liquids. We show that the two sound modes undergo hybridization as a function of interaction strength, which we propose to detect experimentally. This gives a novel insight into the two regimes of Bose gases, defined via the hierarchy of sounds modes.Comment: Main text: 7 pages + 6 figures and Supplementary text: 5 pages + 4 figure

Superfluidity of a laser-stirred Bose-Einstein condensate

We study superfluidity of a cigar-shaped Bose-Einstein condensate (BEC) by stirring it with a Gaussian potential oscillating back and forth along the axial dimension of the condensate, motivated by experiments of C. Raman et al. Phys. Rev. Lett. 83, 2502 (1999). Using classical-field simulations and perturbation theory we examine the induced heating rate, based on the total energy of the system, as a function of the stirring velocity $v$. We identify the onset of dissipation by a sharply increasing heating rate above a velocity $v_c$, which we define as the critical velocity. We show that $v_c$ is influenced by the oscillating motion, the strength of the stirrer, the temperature and the inhomogeneous density of the cloud. This results in a vanishing $v_c$ for the parameters similar to the experiments, which is inconsistent with the measurement of nonzero $v_c$. However, if the heating rate is based on the thermal fraction after a 100 ms equilibration time, our simulation recovers the experimental observations. We demonstrate that this discrepancy is due to the slow relaxation of the stirred cloud and dipole mode excitation of the cloud.Comment: 14 pages + 11 figure

Ectopic expression of the homeobox gene Cdx2 is the key transforming event in a mouse model of t(12;13)(p13;q12) acute myeloid leukemia; a novel mechanism in AML

This thesis demonstrates for the first time that activation of a proto-oncogene by a chromosomal translocation can be the key step in myeloid leukemogenesis, even if a fusion gene is generated and expressed in parallel. This mechanism of AML leukemogenesis was proven in a murine model of t(12;13)(p13;q12) AML, showing that myeloid leukemogenesis is induced by the ectopic expression of Cdx2 and not by the ETV6/CDX2 chimeric gene. Mice transplanted with bone marrow cells retrovirally engineered to express Cdx2 rapidly succumbed to fatal and transplantable AML. In contrast, mice which were transplanted with BM cells expressing the fusion gene alone did not develop AML. The transforming activity of Cdx2 was dependent on an intact homeodomain and the N-terminal transactivation domain. Although mice transplanted with ETV6/CDX2 expressing BM cells did not develop overt disease, these animals suffered from a mild myeloproliferation. Experiments testing the effect of simultaneous expression of the Cdx2 and the fusion gene showed no acceleration or change in phenotype of the disease compared to expression of Cdx2 alone, again demonstrating that the ectopic expression of Cdx2 was the key event in this leukemia model. These findings link proto-oncogene activation to myeloid leukemogenesis, an oncogenic mechanism so far associated mainly with lymphoid leukemias and lymphomas. Our model constitutes the first functional proof that activation of a proto-oncogene by a chromosomal translocation is the key leukemogenic event in AML. As many fusion proteins expressed in AML have clearly no leukemogenic potential in experimental in vivo models, this mechanism might be more important for the development of AML than previously thought and should be included in the pathogenetic models of this disease

The Grading Entropy-Based Criteria for Structural Stability of Granular Materials and Filters

Some rules for particle migration, filtering, and segregation were elaborated on the basis of some simple laboratory tests and data of well-designed, artificial mixtures of natural sand grains. Use was made of the knowledge available in the field and two pairs of grading entropy parameters. These parameters incorporate all information of the grading curve and are pseudo-metrics in the “space of the possible grading curves.

Shapiro steps in driven atomic Josephson junctions

We study driven atomic Josephson junctions realized by coupling two two-dimensional atomic clouds with a tunneling barrier. By moving the barrier at a constant velocity, dc and ac Josephson regimes are characterized by a zero and nonzero atomic density difference across the junction, respectively. Here, we monitor the dynamics resulting in the system when, in addition to the above constant velocity protocol, the position of the barrier is periodically driven. We demonstrate that the time-averaged particle imbalance features a step-like behavior that is the analog of Shapiro steps observed in driven superconducting Josephson junctions. The underlying dynamics reveals an intriguing interplay of the vortex and phonon excitations, where Shapiro steps are induced via suppression of vortex growth. We study the system with a classical-field dynamics method, and benchmark our findings with a driven circuit dynamics.Comment: 7+4 pages, 4+5 figure