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Fracton magnetohydrodynamics
by Marvin Qi, Oliver Hart, Aaron J. Friedman, Rahul Nandkishore, Andrew Lucas
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Submission summary
Authors (as registered SciPost users):  Aaron Friedman · Oliver Hart · Rahul Nandkishore 
Submission information  

Preprint Link:  https://arxiv.org/abs/2205.05695v2 (pdf) 
Date submitted:  20220823 19:22 
Submitted by:  Hart, Oliver 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approach:  Theoretical 
Abstract
We extend recent work on hydrodynamics with global multipolar symmetries  known as "fracton hydrodynamics"  to systems in which the multipolar symmetries are gauged. We refer to the latter as "fracton magnetohydrodynamics", in analogy to conventional magnetohydrodynamics (MHD), which governs systems with gauged charge conservation. We show that fracton MHD arises naturally from higherrank Maxwell's equations and in systems with oneform symmetries obeying certain constraints; while we focus on "minimal" higherrank generalizations of MHD that realize diffusion, our methods may also be used to identify other, more exotic hydrodynamic theories (e.g., with magnetic subdiffusion). In contrast to semimicroscopic derivations of MHD, our approach elucidates the origin of the hydrodynamic modes by identifying the corresponding higherform symmetries. Being rooted in symmetries, the hydrodynamic modes may persist even when the semimicroscopic equations no longer provide an accurate description of the system.
Author comments upon resubmission
List of changes
In response to Report 1:
1) the first paragraph of the introduction has been modified to clarify our use of the term "hydrodynamics"
> "the universal properties of these theories can be characterized within the framework of hydrodynamics, which is the coarsegrained effective theory of the longtime and longwavelength dynamics of systems as they relax to equilibrium"
2) Additionally, we have added a footnote after the sentence ending "...these longlived modes are associated with conserved densities (or Goldstone bosons), and their dynamics is dubbed "hydrodynamics""
> "Note that our use of the term "hydrodynamics"—the coarsegrained description of systems as they relax to equilibrium—does not require that momentum be conserved, and need not correspond to the NavierStokes equations, for example."
3) Above Eq. (3.6), we have explained that the current tensor and the electric field tensor being proportional to one another is, in fact, the most general form of Ohm's law permitted by symmetry, if the electric field tensor transforms in the 5 of SO(3).
4) Similarly, below Eq. (4.7), we have made analogous arguments for the rankn case, where the electric field tensor is a symmetric, traceless, rankn quantity.
5) For the traceful scalar charge theory in Sec. 5, Eqs. (5.5) through (5.7) have been modified to include the two time scales that are in general permitted by SO(3) rotational invariance, since the trace part and the traceless symmetric part can decay on different time scales.
6) We have added the following sentence below Eq. (3.10) to remind the reader that the vanishing of magnetic charge is only ever an approximation in emergent theories:
> "We remind the reader that the vanishing of magnetic charge density can at best only be expected to hold approximately in emergent theories; see Sec. 3.2.3 for a discussion of the length and time scales over which (3.10) provides an accurate description of the dynamics."
7) Below Eq. (3.13) we have added the following sentences to clarify that the structure of timeindependent gauge transformations and the conserved moments of the magnetic field tensor coincide only in the case of selfdual theories:
> "It is worth noting that (3.13) coincides with the structure of timeindependent gauge transformations acting on the vector potential $A_{ij}$, canonically conjugate to $E_{ij}$. This apparent equivalence derives from the selfdual nature of the traceless scalar charge theory—i.e., the derivative and tensor structure of the electric and magnetic Gauss’s laws is identical."
8) The first sentence of Sec. 2.2.5 has been modified in order to define the "magnetic conductivity".
9) The tilde has been removed in (2.21) and in the subsequent paragraph.
10) The sentence in the introduction that states the result for the diffusion constant for rankn theories has been modified to clarify that the result applies only to rankn traceless symmetric theories.
In response to Report 2:
11) The abstract has been modified to better represent the utility of a symmetrybased approach:
> "In contrast to semimicroscopic derivations of MHD, our approach elucidates the origin of the hydrodynamic modes by identifying the corresponding higherform symmetries. Being rooted in symmetries, the hydrodynamic modes may persist even when the semimicroscopic equations no longer provide an accurate description of the system."
12) Similarly, in the introduction, we have emphasized that the main advantage of the "symmetrybased" approach is primarily conceptual in nature:
> "Somewhat surprisingly, a firstprinciples derivation of magnetohydrodynamics using oneform symmetries was not done until the past decade..."
and
> "This approach, based on higherform symmetries, has significant conceptual advantages over more familiar semimicroscopic derivations of MHD. Specifically, the symmetrybased approach highlights the underlying symmetries responsible for the observed longwavelength modes, while also being less limited in its regime of validity than the semimicroscopic approach. For example, in conventional (rankone) MHD, the semimicroscopic derivation invokes approximate separability of the electromagnetic and matter stress tensors. In the symmetrybased approach [48], one invokes hydrodynamic principles to recover a coarsegrained theory of the longtime and longwavelength dynamics of the fields in the most interesting and physically relevant regimes, where there may not be a clean separation between the two tensors. This approach also gives predictions for particular limits of conventional U(1) spinliquids in which the relevant symmetries are weakly broken. In the case of emergent electromagnetism in fractonic spin liquids, the emergent gauge fields are higher rank, leading to additional subtleties and new universality classes. The hydrodynamic description of these higherrank theories is the subject of this work."
13) In the conclusion,
> "The theories we present describe the generic longtime description of the quantum dynamics of fractonic phases (at nonzero charge density) exhibiting gauged—as opposed to global—multipolar symmetries (as relevant to spin liquids and fracton phases). We develop a symmetrybased approach describing arbitrary higherrank theories of this type, and showcase the subdiffusion of magnetic fields as an example of the exotic universal dynamics that may arise in this context."
14) At the top of page 9, we have reminded the reader that SO(3) rotational invariance is assumed throughout the manuscript.
15) Below (2.29), we have explained that spatial inversion can alternatively be used to forbid the term proportional to the vector density, i.e., to require $\alpha = 0$.
In response to Report 3:
[See points 1) and 2) in response to Report 1, which address the manner in which we use the term "hydrodynamics"]
16) To highlight the physical systems that our results apply to, at the start of each section we have made more prominent the citations that discuss concrete examples of lattice models giving rise to emergent electromagnetism of the corresponding rank.
17) Since there are few examples in the literature, we have added a new subsection [Section 4.2] in which we illustrate an emergent rankthree variant of QED in a lattice O(2) quantum rotor system.
18) We have also included an additional figure (Fig. 2) to accompany the introduction of the rankthree lattice model.
[See points 3), 4), 5) in response to Report 1, which employ a symmetrybased analysis to constrain the form of Ohm's law]
Current status:
Reports on this Submission
Anonymous Report 3 on 202298 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2205.05695v2, delivered 20220908, doi: 10.21468/SciPost.Report.5666
Report
I thank the authors for addressing my points. However, I still think the revised version does not convincingly justify the use of the name “fracton magnetohydrodynamics” in the title. Even in the view of modern developments I don’t think diffusion is referred to as hydrodynamics. For example in the recent book Relativistic Fluid Dynamics In and Out of Equilibrium by Paul Romatschke and Ulrike Romatschke diffusion is referred to as an analogy to fluid dynamics. Also modern developments in magnetohydrodynamics based on twoform symmetries focus on systems with momentum conservation. Fractons admit a hydrodynamic description with and without momentum conservation. This leads to the conclusion that calling diffusion or subdiffusion (magneto)hydrodynamics leads to confusion. This is the case even if the authors can write in the text what they really mean. As a result I think that the title suggests more than the authors actually do and, in my opinion it has to be changed, to avoid misleading the reader. If the authors think that using the term magnetic diffusion is inappropriate they can suggest another name but certainly not “fracton magnetohydrodynamics”.
Requested changes
1. Change the tile and possibly avoid abusing the term magnetohydrodynamics.
Report
The authors answered my questions satisfactorily. I agree with referee three that the paper lacks concrete physical systems. However, the article provides a theoretical prescription for higherrank hydrodynamics. I recommend this paper be published on SciPost Physics.