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Quantum BoseFermi droplets
by Debraj Rakshit, Tomasz Karpiuk, Mirosław Brewczyk, Mariusz Gajda
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Submission summary
Authors (as registered SciPost users):  Miroslaw Brewczyk · Mariusz Gajda · Tomasz Karpiuk · Debraj Rakshit 
Submission information  

Preprint Link:  https://arxiv.org/abs/1801.00346v3 (pdf) 
Date submitted:  20181130 01:00 
Submitted by:  Brewczyk, Miroslaw 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We study stability of a zero temperature mixture of attractively interacting degenerate bosons and spinpolarized fermions in the absence of confinement. We demonstrate that higher order corrections to the standard mean field energy of the system can lead to a formation of BoseFermi liquid droplets  selfbound incompressible systems in a threedimensional space. The stability analysis of the homogeneous system is supported by numerical simulations of finite systems by explicit inclusion of surface effects. Our results indicate that BoseFermi droplets can be realized experimentally.
Current status:
Reports on this Submission
Anonymous Report 2 on 20181221 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1801.00346v3, delivered 20181221, doi: 10.21468/SciPost.Report.763
Strengths
1. The current manuscript certainly touches on a highly topical and interesting area. A feasible approach to dilute quantum droplets in free space was presented in just 2015 (for BoseBose mixtures [1]). Dilute dipolar gas droplets were observed in 2016 [2], and droplets of BoseBose mixtures very recently [34].
2. This submission appears to be the first work considering selfbound BoseFermi mixtures stabilized by LeeHuangYang effects with only contact interactions.
3. The submission is clearly laid out.
Weaknesses
1. The use of a single complex function to represent all of the fermions in this system is not physically reasonable. There are significant issues both for the ground state and for dynamics.
Report
1. Ground state
In a dilute selfbound system there do not tend to be many bound states. For example Fig. 3(a) of [5] identifies in the order of 10 modes for fermionic impurities in a dipolar Bose droplet, and [1] identifies only a few modes for a BoseBose mixture. At most one fermion can occupy each bound state. Any additional fermions in addition to those in bound modes will occupy the continuum and escape from the mixture. Since the number of bound modes is small, each of them should be treated discretely, as [5] did for fermions trapped in a dipolar Bose droplet.
There will be a balance. The system needs to be deeply selfbound to have enough discrete modes below the energy cutoff at the continuum, but not so deeply bound that the density is too high. The loss from threebody interactions would then make the timescales unfeasible in experiment.
2. Dynamics
The phase of a single Fermi wavefunction cannot generally be used to represent the dynamics of the fermions, as shown by Ref. [6] (with mapping from bosons).
The submission cites three papers related to the single Fermi wavefunction, two on very different systems (metal clusters and a single helium atom) and Ref. [7]. The only comparison to experiment or other methods in Ref. [7] seems to be Fig. 1. The difference of the spin dipole mode from one (the noninteracting limit) is either small or the difference from one is comparable to the difference from experiment.
[1] D. S. Petrov, Phys. Rev. Lett. 115, 155302 (2015)
[2] M. Schmitt et al, Nature 539, 259 (2016)
[3] C. R. Cabrera et al, Science 359, 301 (2018)
[4] G. Semeghini et al, Phys. Rev. Lett. 120, 235301 (2018)
[5] M. Wenzel et al, Phys. Scr. 93 104004 (2018)
[6] K. K. Das, G. J. Lapeyre, and E. M. Wright, Phys. Rev. A 65, 063603 (2002)
[7] P. T. Grochowski et al, Phys. Rev. Lett. 119, 215303 (2017)
Requested changes
1. Use a methodology that allows for the small number of discrete modes available to the fermions.
2. With that methodology show the bounds on the number of bosons and fermions for a droplet to exist and how that varies with interaction strength.
3. For reasonable interaction strengths, then give corresponding peak densities in SI units to allow comment on threebody loss.
Author: Mariusz Gajda on 20181228 [id 394]
(in reply to Report 2 on 20181221)
The referee admits that our manuscript touches on a highly topical and interesting area, but the use of a single complex function to represent all of the fermions in this system is not physically reasonable. There are significant issues both for the ground state and for dynamics.
We agree that our approach might have weaknesses in a case of small systems of tens fermions as studied in [1]. Discreetness of fermionic states might be an issue then. We have done orbital calculations in such a situation recently. The differences are not big, and we are now able to revise the manuscript in this direction. However, orbital calculations are numerically very demanding, thus must be limited to small droplets only.
But as shown in [2] not only few fermions, but also about 400 fermionic atoms can be trapped in a bosonic cloud. Then the mean field approach is applicable.
The mean field approach used by us and rigorously transformed to the pseudo wavefunction formalism (with some limitations on the velocity field) corresponds to the limit of a large number of particles. Our goal was to compare results of the first part of our manuscript, where we consider infinite system, to the results which account for the surface effects. We choose big systems then.
In a stationary case, the velocity field is zero, and there is no problem in transforming the hydrodynamic description into the pseudo wavefunction formalism. As the dynamic situation is concerned, we study adiabatic opening of trap. As can be checked a posteriori, no vorticity is created then. Hydrodynamic approach and pseudo wavefunction formalisms are equivalent.
In our description the fermion contribution to the mean field energy is given by the ThomasFermi functional with the Weizscaker term. The functional used is very similar to the one introduced by Thomas and Fermi. It is well known that binding energies of multielectron atoms given by the ThomasFermi model differ from the `exact ones’ by about 10%. Differences are the largest for atoms with closed shells, and become smaller with increasing atomic number. Thus, by analogy, we expect that our approach is quite accurate for larger system such as we study in the manuscript. Orbital based calculations are beyond our reach for these number of atoms.
[1] Matthias Wenzel, Tilman Pfau and Igor FerrierBarbut, Physica Scripta 93, 104004 (2018)
[1] B. J. DeSalvo, Krutik Patel, Jacob Johansen, and Cheng Chin, PRL 119, 233401 (2017)
Anonymous Report 1 on 20181218 (Invited Report)
 Cite as: Anonymous, Report on arXiv:1801.00346v3, delivered 20181218, doi: 10.21468/SciPost.Report.751
Strengths
1 The topic is interesting and timely.
Weaknesses
1The methods used are not valid to describe fermion dynamics.
2The methods used are not valid to describe fermions in selfbound droplets.
Report
This manuscript theoretically investigates the possibility of selfbound BoseFermi droplets. It begins with an infinite uniform system analysis then moves to the finite selfbound droplet. Later, dynamic simulations are considered to test droplet stability, and droplet preparation. I do not recommend this manuscript for publication because the methods employed are inappropriate to describe the system studied.
Selfbound droplets typically support only a small number of bound states, usually a tiny percentage of the total atom number, see Petrov, PRL 115, 155302 (2015), Wenzel et al., Physica Scripta 93, 104004 (2018), and Baillie et al., PRL 119, 255302 (2017). While this is not a problem for the bosons, only one fermion can occupy each bound state. If the number of fermions exceeds the number of bound states then the excess fermions must be unbound, which will affect the stability of the entire droplet. The model in the present manuscript doesn’t account for this crucial physics, nor do they consider how the ratio of fermion number to bound state number behaves in the thermodynamic limit. Therefore, the authors cannot claim that their results are physically relevant for either the finite or uniform systems. Such considerations must be a central focus for any paper considering the selfbound nature of BoseFermi droplets, as was demonstrated in the paper on BoseFermi droplets with dipolar interactions by the Pfau group: Physica Scripta 93, 104004 (2018).
Furthermore, for the section on finite systems, the small numbers of fermions expected (typically 10s of bound states) should instead be treated as discrete modes, as was done in the paper by the Pfau group: Physica Scripta 93, 104004 (2018). Even if the fermion number was large, any continuous treatment (e.g. local density approximation) of the fermions would require a high energy cutoff (at zero energy) so that only the bound fermion states are included within the droplet.
Another significant problem is that Eq. (10) is inappropriate to describe the dynamics of fermions. Equation (10) attempts to describe all fermions within a single pseudo wavefunction. However, it is wellknown that this is unphysical for describing dynamics. What would be the physical interpretation of the ‘phase’ of the fermion wavefunction? The unphysical effects on the dynamics from assigning a ‘phase’ to a single wavefunction to describe many fermions has already been clearly addressed in the literature. For example, the paper by Girardeau et al. PRL 84, 5239 (2000), which considers fermionized bosons, focusses quite a lot on this point. For another example see Kunal et al. PRA 65, 063603 (2002).
Requested changes
(see report)
Author: Miroslaw Brewczyk on 20181226 [id 393]
(in reply to Report 1 on 20181218)
Below we shortly reply to the referee's criticism:
 "The methods used are not valid to describe fermion dynamics"
Our explanation must be not clear enough. Contrary to what the referee says, we do not assign a single wave function to a manyfermion system.
We use a hydrodynamic approach originated in the density functional method. The hydrodynamic equations for fermionic system can be rigorously derived based on quantum kinetic equations for reduced density matrices. A 3component velocity field and an atomic density are used to describe the system.
If the velocity field is a gradient of a scalar potential then problem can be simplified because of reduction of the number of dynamical variables. The density (in fact its square root) and the velocity potential can be combined into a single complex function. This way the hydrodynamic equations, under some assumptions, can be mapped to a pseudowave function dynamics.
In a case of violent dynamics and turbulent flow one must use all components of the velocity field. This is because no potential can be assigned to the velocity field then. We do not study such a situation.
We show that the BoseFermi droplet can be created dynamically by slow opening of a trap. We have checked (and results might be included into the manuscript) that the atomicorbital approach and a hydrodynamic description give very similar results already for a droplet with tens of fermions and ten times larger number of bosons. Larger systems are not tractable by this method.
 "The methods used are not valid to describe fermions in selfbound droplets."
In the first part we study infinite system, and we simply minimize the energy functional under physical constraints.
To account for the surface effects we modified the energy functional. Moreover in stationary case the velocity field vanishes, and our hydrodynamic description of fermions becomes the ThomasFermi model with the Weizsacker correction with all necessary modifications like replacing the Coulomb potential of nucleus by the effective potential produced by bosons.
The ThomasFermi model, used to describe multielectron atoms, has some drawbacks. It does not account for shell effects, but gives quite reasonable estimations of basic characteristics of atoms with atomic number Z. There is no better approach in the limit of infinite system than the density functional method. This is what we do for large system, being aware of limitations. We cannot do any better.
Referee correctly noticed that in a case of small systems the orbital approach is the one which is more adequate.
We have recently applied the atomicorbital approach to a small system of BoseFermi mixture and confirmed the existence of BoseFermi droplets. These results can be incorporated into the present paper if we are given a chance.
 "Another significant problem is that Eq. (10) is inappropriate to describe the dynamics of fermions."
We claim that Eqs. (10) are good enough to describe adiabatic formation of the BoseFermi droplet. We do not claim that every dynamics can be described by Eqs. (10).
As we have already mentioned, the Madelung approach is equivalent to the hydrodynamic description if the velocity field is irrotational. However, this is not a general feature. For example, when vortices are present in the system, the phase is not differentiable at the vortices cores and the equivalence between the Madelung and hydrodynamic approaches is broken.
Finally, the paper of Girardeau et al. mentioned by the referee is related to the interference effect studied for fermionized bosons. We are well aware of this paper. Here, two interfering clouds of fermions moving irrotationally can not be, in general, described as a single fluid with a potential flow since the total velocity is calculated as a ratio of the total density current to the total density.
However, the formation of the BoseFermi droplets has nothing to do with creation of vortices nor with any subtleties of nonpotential flow as the one studied by Girardeau et al. (the fermionic cloud is simply connected).
Author: Debraj Rakshit on 20190302 [id 455]
(in reply to Miroslaw Brewczyk on 20181226 [id 393])
We thank for the comment. Indeed the paper of S. K. Adhikari considers related, but different problem, and if we have an opportunity we shall add some critical comments in the present manuscript. We did not include the corresponding citation in the first version of our paper simply because our manuscript appeared on arXiv on 01 January 2018, well ahead of the S.
K. Adhikari's paper  sumbitted for publication much later.
Anonymous on 20190226 [id 449]
(in reply to Miroslaw Brewczyk on 20181226 [id 393])
I am interested in the topic of this study and I find the comments by the referees to this manuscript of interest. One of the referees commented that
" This submission appears to be the first work considering selfbound BoseFermi mixtures stabilized by LeeHuangYang effects with only contact interactions."
This statement is not to the point. I find a very similar work (S K Adhikari 2018 Laser Phys. Lett. 15 095501) demonstrating the existence and studying the properties of selfbound BoseFermi mixtures stabilized by LeeHuangYang effects with only contact interactions using essentially the same Hamiltonian and same analytical and numerical procedures. Moreover, that paper goes beyond and considers the effect of threebody force on these states. This paper was published long before this manuscript was submitted for publication. The authors should clearly acknowledge this fact in the Abstract and Introduction of the manuscript and point out what is new or different in their study for the benefit of the reader.
Author: Miroslaw Brewczyk on 20190117 [id 406]
(in reply to Report 2 on 20181221)We performed further calculations by using the HartreeFock method (in reply to the requested change number 1) and in the attached file we compare the results with the ones included already in the main text, obtained within the hydrodynamic approach.
Attachment:
comment.pdf
Matthew Davis on 20190205 [id 427]
(in reply to Miroslaw Brewczyk on 20190117 [id 406])Following on from your additional calculations, below I relay further specific comments from one of the referees of your initial submission. I will hope this will help expedite the next round of refereeing:
As the selfbound droplet is not trapped, the effective potential is finite and the number of bound states is limited before the continuum is reached. Can you please calculate the number of bound states of the bosons (i.e. the states available to the fermions)? A full Bogoliubovde Gennes calculation would be time consuming, but a possible simple and approximate approach would be to take the effective potential of the bosons in the selfbound Bose+Fermi droplet from your calculations and find the number of noninteracting modes supported by that numerical effective potential.
Can you please perform the same calculation as in your reply of 17 Jan, but applied to the solid curve in Fig. 3 of your manuscript, where there is expansion of the gas. A smaller atom number would be OK.