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The origin of the period$2T/7$ quasibreathing in diskshaped GrossPitaevskii breathers
by J. Torrents, V. Dunjko, M. Gonchenko, G. E. Astrakharchik, M. Olshanii
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Submission summary
Authors (as registered SciPost users):  G. E. Astrakharchik · Vanja Dunjko · Maxim Olshanii 
Submission information  

Preprint Link:  scipost_202108_00053v2 (pdf) 
Date accepted:  20220202 
Date submitted:  20211116 04:38 
Submitted by:  Dunjko, Vanja 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Computational 
Abstract
We address the origins of the quasiperiodic breathing observed in [Phys. Rev.\ X vol. 9, 021035 (2019)] in diskshaped harmonically trapped twodimensional Bose condensates, where the quasiperiod $T_{\text{quasibreathing}}\sim$~$2T/7$ and $T$ is the period of the harmonic trap. We show that, due to an unexplained coincidence, the first instance of the collapse of the hydrodynamic description, at $t^{*} = \arctan(\sqrt{2})/(2\pi) T \approx T/7$, emerges as a `skillful impostor' of the quasibreathing halfperiod $T_{\text{quasibreathing}}/2$. At the time $t^{*}$, the velocity field almost vanishes, supporting the requisite timereversal invariance. We find that this phenomenon persists for scaleinvariant gases in all spatial dimensions, being exact in one dimension and, likely, approximate in all others. In $\bm{d}$ dimensions, the quasibreathing halfperiod assumes the form $T_{\text{quasibreathing}}/2 \equiv t^{*} = \arctan(\sqrt{d})/(2\pi) T$. Remaining unresolved is the origin of the period$2T$ breathing, reported in the same experiment.
List of changes
1. Fixed Eq. (2) to address Referee 2's requested change 1, namely, we removed the proportionality constant $C$ and instead used the proportionality symbol $\propto$. Also, we removed the braces.
2. Corrected a typo in the second equation in Eq. (5), namely the missing velocity field after the del operator on the lefthand side. This was Referee 1's requested change 1 and Referee 2's requested change 2.
3. The powerlaw equation of state is now numbered as Eq. (8).
4. After Eq. 8 (formerly 7), we clarified that the DamskiChandrasekhar shock wave solution is approximate and valid at the initial stages of propagation.
5. Right before Eq. (12) (formerly 11), we explained that $V_{\text{inner}}(t) \equiv \dot{R}_{\text{inner}}(t)$ and
$V_{\text{outer}}(t) \equiv \dot{R}_{\text{outer}}(t)$. This addresses Referee 2's requested change 12.
6. Right after Eq. (12) (formerly 11), fixed the notation of the limit of delta t, as reqested by Referee 2's requested change 4.
7. After Eq. (12), we added an explanation of why Eq. (9) (formerly 8) is an approximate solution of Eq. (6) for d>1, as reqested by Referee 2's requested change 3.
8. In Eq. (12) (formerly 12), in the last equation, corrected 'for $R_{\text{outer}}$' to read 'for $r>R_{\text{outer}}$'. This was Referee 2's requested change 5.
9. After Eq. (17) (formerly 16), we added an explanation that $N$, the number of particles, is a volume integral over $n(r,t)$. This addresses Referee 2's requested change 7.
10. We added a substantial amount of detail to the proof of Statemet 5, that the singlevaluedness of the velocity field implies that the velocity at the origin is zero. This addresses Referee 2's requested change 8.
11. In the expression for $1/r$, below Eq. (24) (formerly 23), we replaced all appeances of $R_{\text{inner}}(t)$ by $\bar{R}_{\text{inner}}(t)$. This addresses Referee 2's requested change 9.
12. In the caption to to Fig. 1, we corrected the way we refer to Eqs. (29) and (35) (formerly 28 and 34). This addresses one part of Referee 2's requested change 10.
13. At the end of the caption to Fig. 1, we added an explanation of why the hydrodynamical equations cannot be propagated past $t^{∗}$. This addresses the other part of Referee 2's requested change 10.
14. In the text of Statement 7, we clarified that the statement holds for powerlaw equations of state. This addresses one part of Referee 2's requested change 11.
15. In the text of Statement 8, we also clarified that the statement holds for powerlaw equations of state. This addresses the other part of Referee 2's requested change 11.
16. After the second displayed equation in Sec. 12, we added a statement explaining that the relevant velocity scale to compare the velocity field with is the initial speed of sound. This addresses Referee 2's requested change 6.
17. Near the end of the page where Sec. 12 begins, we corrected $2T/7 = 0.296\ldots \times T$ to $2T/7 = 0.286\ldots \times T$. This addresses Referee 1's requested change 2.
18. In the caption of Fig. 2, we changed the description of lines from 'dotted' and 'dashed' to 'shortdashed' and 'longdashed'. This addresses Referee 2's requested change 13.
19. At the end of Sec. 12, we wrote down the GrossPitaevskii equation, with an explanation of the relevant parameters. We emphasized that the healing length and the number of particles were chosen to be close to the values they had in the ENS experiment [Phys. Rev. X vol. 9, 021035 (2019)]. This addresses Referee 2's requested change 15.
20. At the end of Sec. 12, we added a sentence explaining why we decided against attempting to introduce a quantummechanical analogue of the local velocity. This addresses Referee 2's requested change 14.
Published as SciPost Phys. 12, 092 (2022)
Reports on this Submission
Report
It is my impression that the points raised in my first report have been adequately taken into account in the revised manuscript. I have no further "urgent" comments.
Author: Maxim Olshanii on 20220103 [id 2063]
(in reply to Report 1 on 20211121)Without all the hard work by the Referee 1, this article would not happen: thank you.
Author: Maxim Olshanii on 20220103 [id 2062]
(in reply to Report 2 on 20211212)We are grateful to the Referee 2 for all the help with the manuscript.