Comment on "Dislocation Structure and Mobility in hcp 4 He"
aa r X i v : . [ c ond - m a t . o t h e r] S e p Comment on “Dislocation Structure and Mobil-ity in hcp He”
In their Letter [1], Borda, Cai, and de Koning reportthe results of ab initio simulations of dislocations respon-sible for the giant plasticity [2]. The authors claim keyinsights into the recent interpretations of (i) the giantplasticity and (ii) the mass flow junction experiments.The purpose of this Comment is clarifying the role of dis-locations in the mass flow in conjunction with explainingthat the part (ii) of the claim is misleading.Borda et al. find that their dislocations do not havesuperfluid cores. This fact, however, is not crucial for theinterpretation of the mass supertransport, including theeffect of giant isochoric compressibility (aka the syringeeffect) [3]. Furthermore, the fact is not even new.The first-principle theoretical results showing that cer-tain (!) screw and edge dislocations feature superfluidcores were reported in Refs. [4] and [5], respectively. Also,the behavior of the edge dislocation of Ref. [5] was foundto be consistent with the phenomenon of superclimb. Inits turn, the superclimb remains the only known under-lying mechanism behind the syringe effect. Apart frombeing fundamentally interesting on its own, the syringeeffect is central for the liquidless supertransport setups[6]. Thus, any theory aiming at explaining the super-transport through solid must account for the syringe ef-fect too. The results of Ref. [4, 5] provide a consistent,and up to now unique, first-principle basis for interpret-ing all known supertransport-related phenomena in solid He [3, 6–8].As is known, dislocations are characterized by orien-tation of their core and the Burgers vector. The dislo-cations studied in Ref. [1]—with the core and Burgersvector both along basal plane—are different from thosefound to have superfluid core [4, 5]—with the Burgersvector perpendicular to the basal plane and core eitherperpendicular to the plane [4] or along the plane [5].Hence, the statement that (Qt) “the interpretation of re-cent mass flow experiments in terms of a network of 1DLuttinger-liquid systems in the form of superfluid dislo- cation cores does not involve basal-plane dislocations”made in Ref. [1] is misleading.Moreover, that cores of dislocations with the Burg-ers vector along basal plane (studied in Ref. [1]) are notsuperfluid has been emphasized in Refs. [4, 9, 10]: inthe caption to Fig. 6 in Ref. [9]; on p.1 at the end ofthe 1st and beginning of the 2nd column and on p.3 ofRef. [4], (Qt) “[In the case of edge dislocations, this pro-tocol leads to an insulating ground state.]”; in the lastparagraph on p.3 of Ref. [10]. Likewise, the effect ofsplitting into partials—claimed in Ref. [1] as a new andcrucial observation—has been reported in Refs. [5, 10] forthe edge dislocations of both types—with (in the section“Numerical results” on p.3 of Ref. [5]) and without super-fluid cores (in the the last paragraph on p.3 of Ref. [10]).Furthermore, the energy of the structural fault found inRef. [5] to be much smaller than any other typical en-ergy scale in solid He implies large splitting of disloca-tions with core in the basal plane. Thus, Ref. [1] neithernegates the results of Refs. [4, 5, 9, 10] nor provides newinsights into superfluidity of dislocations.Finally, it is worth noting that the observation of nosuperfluidity in Ref. [1] lacks not only novelty but, mostlikely, also the control of the numerical data. While deal-ing with large systems, the authors do not use worm up-dates [11]. In such a case, PIMC algorithm is known tobe notoriously prone to non-ergodicity in the worldlinewinding number space, so that the absence of macro-scopic permutation cycles could merely reflect the non-ergodicity of the scheme rather than the absence of su-perfluidity.We thank Nikolay Prokof’ev for useful discussions andacknowledge support from the National Science Founda-tion under the grants PHY-1314469 and PHY-1314735.
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