| 1. |
- Agterberg, Daniel F., et al.
(författare)
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Microscopic prediction of skyrmion lattice state in clean interface superconductors
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 90:6, s. 064509-
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Tidskriftsartikel (refereegranskat)abstract
- When an in-plane field is applied to a clean interface superconductor, a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO)-like phase is stabilized. This phase has a U(1)xU(1) symmetry and, in principle, this symmetry allows for flux carrying topological excitations different from Abrikosov vortices (which are the simplest defects associated with S-1 --> S-1 maps). However, in practice, largely due to electromagnetic and other intercomponent interactions, such topological excitations are very rare in superconducting systems. Here, we demonstrate that a realistic microscopic theory for interface superconductors, such as SrTiO3/LaAlO3, predicts an unconventional magnetic response where the flux-carrying objects are skyrmions, characterized by homotopy invariants of S-2 --> S-2 maps. Additionally, we show that this microscopic theory predicts that stable fractional vortices form near the boundary of these superconductors. It also predicts the appearance of type-1.5 superconductivity for some range of parameters. Central to these results is the assumption that the Rashba spin-orbit coupling is much larger than the superconducting gap.
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| 2. |
- Babaev, Egor, et al.
(författare)
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A superconductor to superfluid phase transition in liquid metallic hydrogen
- 2004
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 431:7009, s. 666-668
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Tidskriftsartikel (refereegranskat)abstract
- Although hydrogen is the simplest of atoms, it does not form the simplest of solids or liquids. Quantum effects in these phases are considerable (a consequence of the light proton mass) and they have a demonstrable and often puzzling influence on many physical properties(1), including spatial order. To date, the structure of dense hydrogen remains experimentally elusive(2). Recent studies of the melting curve of hydrogen(3,4) indicate that at high (but experimentally accessible) pressures, compressed hydrogen will adopt a liquid state, even at low temperatures. In reaching this phase, hydrogen is also projected to pass through an insulator-to-metal transition. This raises the possibility of new state of matter: a near ground-state liquid metal, and its ordered states in the quantum domain. Ordered quantum fluids are traditionally categorized as superconductors or superfluids; these respective systems feature dissipationless electrical currents or mass flow. Here we report a topological analysis of the projected phase of liquid metallic hydrogen, finding that it may represent a new type of ordered quantum fluid. Specifically, we show that liquid metallic hydrogen cannot be categorized exclusively as a superconductor or superfluid. We predict that, in the presence of a magnetic field, liquid metallic hydrogen will exhibit several phase transitions to ordered states, ranging from superconductors to superfluids.
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| 3. |
- Babaev, Egor
(författare)
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Andreev-Bashkin effect and knot solitons in an interacting mixture of a charged and a neutral superfluid with possible relevance for neutron stars
- 2004
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Ingår i: Physical Review D. Particles and fields. - 0556-2821 .- 1089-4918. ; 70:4
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Tidskriftsartikel (refereegranskat)abstract
- We discuss a mixture of interacting neutral and charged Bose condensates, which is supposed being realized in the interior of neutron stars in the form of a coexistent neutron superfluid and protonic superconductor. We show that in this system, besides ordinary vortices of the S-1-->S-1 map, the neutron condensate also allows for (meta)stable finite-length knotted solitons, which are characterized by a nontrivial Hopf invariant and in some circumstances may be stabilized by a Faddeev-Skyrme term induced by the drag effect. We also consider a helical protonic flux tube in this system and show that, in contrast, it does not induce a Faddeev-Skyrme term.
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| 4. |
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| 5. |
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| 6. |
- Babaev, Egor, et al.
(författare)
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Comment on "Ginzburg-Landau theory of two-band superconductors : Absence of type-1.5 superconductivity"
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:1, s. 016501-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The recent paper by V. G. Kogan and J. Schmalian [Phys. Rev. B 83, 054515 (2011)] argues that the widely used two-component Ginzburg-Landau (GL) models are not correct, and further concludes that in the regime which is described by a GL theory there could be no disparity in the coherence lengths of two superconducting components. This would in particular imply that [in contrast to U(1) x U(1) superconductors] there could be no "type-1.5" superconducting regime in U(1) multiband systems for any finite interband coupling strength. We point out that these claims are incorrect and based on an erroneous scheme of reduction of a two-component GL theory.
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| 7. |
- Babaev, Egor
(författare)
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Dual neutral variables and knot solitons in triplet superconductors
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 88, s. 177002-
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Tidskriftsartikel (refereegranskat)abstract
- We derive a dual presentation of a free energy functional for spin-triplet superconductors in terms of gauge-invariant variables. The resulting equivalent model in ferromagnetic phase has a form of a version of the Faddeev model. This allows one, in particular, to conclude that spin-triplet superconductors allow formation of stable finite-length closed vortices (knotted solitons).
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| 8. |
- Babaev, Egor
(författare)
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Fractional-flux vortices and spin superfluidity in triplet superconductors
- 2005
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 94:13
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Tidskriftsartikel (refereegranskat)abstract
- We discuss a novel type of fractional-flux vortices along with integer flux vortices in Kosterlitz-Thouless transitions in a triplet superconductor. We show that under certain conditions a spin-triplet superconductor should exhibit a novel state of spin superfluidity without superconductivity.
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| 9. |
- Babaev, Egor
(författare)
-
From exotic superconductivity to QCD and back again searching things in common
- 2001
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is devoted largely to two new features which Particle Physics and Condensed Matter theory have in common. The first part of the thesis is a discussion of formation of phases precursory to the chiral phase transition in relativistic models with four-fermion interactions, considered as toy models for QCD. This discussion is based on analogy with the novel pseudogap concept in strong-coupling and low carrier density superconductors. In the second part we discuss a condensed mater realizations of knotted solitions, which were discussed earlier by Faddeev and Niemi. We argue that these defects can exist in superconducting liquid metallic hydrogen and deuterium and two-band superconductors. The knotted solitions is a novel for condensed matter concept and thus it is of great academic interest in this field. The macroscopic quantum origin of topological defects in superfluids implies its rather direct observability. This possibility to actually observe these objects, their properties, stability and interactions may result in a 'feedback" for understanding its role in the infrared limit of Yang-Mills theory complementing ongoing numerical simulations. In the third part of the thesis we study the effect of Coulumb interaction on formation of charged vortices in the Abelian Higgs model in the presence of a uniform compensating background field. This study is also of interdisciplinary interest. The description of granular superconductors and Josephson junction arrays usually includes Coulomb terms which accounts for the presence of the cristal lattice. Also it may be relevant for extreme strong coupling or low carrier density superconductors which may have vortices which r-ores are not filled by decomposed Cooper pairs. This study also allows to estimate characteristic length scales of a charged vortices which should be of interests for the studies of knotted solitons formed by two complex scalar fields with Coulomb interaction discussed by Faddeev and Niemi.
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| 10. |
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| 11. |
- Babaev, Egor, et al.
(författare)
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Hidden symmetry and knot solitons in a charged two-condensate Bose system
- 2002
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 65:10
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Tidskriftsartikel (refereegranskat)abstract
- We show that a charged two-condensate Ginzburg-Landau model or equivalently a Gross-Pitaevskii functional for two charged Bose condensates, can be mapped onto a version of the nonlinear O(3) sigma model. This implies in particular that such a system possesses a hidden O(3) symmetry and allows for the formation of stable knotted solitons.
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| 12. |
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| 13. |
- Babaev, Egor, et al.
(författare)
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Hydrodynamics of Borromean Counterfluids
- 2024
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Ingår i: Physical Review Letters. - : American Physical Society (APS). - 0031-9007 .- 1079-7114. ; 133:2
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Tidskriftsartikel (refereegranskat)abstract
- Counterflow superfluidity in a system with N≥3 components is distinctively different from the N=2 case. The key feature is the difference between the number (N) of elementary vortex excitations and the number (N-1) of independent branches of phonon modes, that is, the number of superfluid modes is larger than the number of ordered phase variables. We formulate a hydrodynamic theory of this state. We show how all the dynamical and statistical aspects of this ("Borromean") type of ordering are naturally described by effective N-component theory featuring compact-gauge invariance. We also discuss how off diagonal intercomponent couplings convert the Borromean supercounterfluid into a Borromean insulator, with an emphasis on the properties of a nontrivial state with broken time-reversal symmetry.
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| 14. |
- Babaev, Egor, et al.
(författare)
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Magnetic Field Delocalization and Flux Inversion in Fractional Vortices in Two-Component Superconductors
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:23
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate that, in contrast with the single-component Abrikosov vortex, in two-component superconductors vortex solutions with an exponentially screened magnetic field exist only in exceptional cases: in the case of vortices carrying an integer number of flux quanta and in a special parameter limit for half-quantum vortices. For all other parameters, the vortex solutions have a delocalized magnetic field with a slowly decaying tail. Furthermore, we demonstrate a new effect which is generic in two-component systems but has no counterpart in single-component systems: on exactly half of the parameter space of the U(1)xU(1) Ginzburg-Landau model, the magnetic field of a generic fractional vortex inverts its direction at a certain distance from the vortex core.
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| 15. |
- Babaev, Egor
(författare)
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Mass generation without symmetry breakdown in the chiral Gross-Neveu model at finite temperature and finite N in 2+1 dimensions
- 2001
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Ingår i: Physics Letters B. - 0370-2693 .- 1873-2445. ; 497:04-mar, s. 323-327
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Tidskriftsartikel (refereegranskat)abstract
- The chiral Gross-Neveu model is one of the most popular toy models for QCD being a generic testing field for many ideas in particle physics. It has been studied in the past in detail in the limit of infinite number of flavors of fermions. Quite astonishingly, the study of this model was not carried through in all its facets. The most important omission is the study of the onset of quasi-long-range order in the decoupled massless phase field. The present work eliminates this deficiency. In this paper we derive behavior of the Kosterlitz-Thouless transition in this model at finite temperature in 2 + 1 dimensions in the regime when the number N of field components is large but finite. We also prove the anticipated before key feature of the model, namely, that in the regime of infinite N the temperature of the Kosterlitz-Thouless transition merges with the critical temperature T*, given by a mean-field equation for the gap modulus, thus recovering the BCS-like scenario [(T* - T-KT)/T* --> 0] of the phase transition at N --> infinity.
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| 16. |
- Babaev, Egor
(författare)
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Non-Meissner electrodynamics and knotted solitons in two-component superconductors
- 2009
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 79:10
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Tidskriftsartikel (refereegranskat)abstract
- I consider electrodynamics and the problem of knotted solitons in two-component superconductors. Possible existence of knotted solitons in multicomponent superconductors was predicted several years ago. However, their basic properties and stability in these systems remain an outstandingly difficult question both for analytical and numerical treatment. Here I propose a special perturbative approach to treat self-consistently all the degrees of freedom in the problem. I show that there exists a length scale for a Hopfion texture where the electrodynamics of a two-component superconductor is dominated by a self-induced Faddeev term, which is in stark contrast to the Meissner electrodynamics of single-component systems. I also show that at certain short length scales knotted solitons in the two-component Ginzburg-Landau model are not described by a Faddeev-Skyrme-type model and are unstable. However, these solitons can be stable at some intermediate length scales. I argue that configurations with high topological charge may be more stable in these systems than low-charge configurations. In the second part of the paper I discuss qualitatively different physics of the stability of knotted solitons in a more general Ginzburg-Landau model and point out the physically relevant terms which enhance or suppress the stability of knotted solitons. With this argument it is demonstrated that Ginzburg-Landau models possess stable knotted solitons.
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| 17. |
- Babaev, Egor
(författare)
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Nonlinear sigma model approach for chiral fluctuations and symmetry breakdown in the Nambu-Jona-Lasinio model
- 2000
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Ingår i: Physical Review D. Particles and fields. - 0556-2821 .- 1089-4918. ; 6207:7
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Tidskriftsartikel (refereegranskat)abstract
- Tills paper is organized in two parts. We start with the observation that the recent claim that the chiral symmetry in the Nambu-Jona-Lasinio (NJL) model is necessarily restored by violent chiral fluctuations at N-c = 3 [H. Kleinert and B. Van den Bossche, Phys. Lett. B 474, 336 (2000)] appears to be incorrect since the critical stiffness of the effective nonlinear sigma model used in the above reference is not a universal quantity in 3 + 1 dimensions. In the second part we discuss a modified Nn model, where the critical stiffness is expressed via an additional cutoff parameter. This model displays a symmetry breakdown, and also under certain conditions the chiral fluctuations give rise to a phase analogous to pseudogap phase of superconductors with strong coupling or low carrier density.
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| 18. |
- Babaev, Egor
(författare)
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Nonlinear sigma model approach for phase disorder transitions in chiral Gross-Neveu, Nambu-Jona-Lasinio models and strong-coupling superconductors
- 2001
-
Ingår i: International Journal of Modern Physics A. - 0217-751X .- 1793-656X. ; 16:7, s. 1175-1197
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Forskningsöversikt (refereegranskat)abstract
- We briefly review the nonlinear sigma model approach for the subject of increasing interest: two-step phase transitions in the Gross-Neveu and the modified Nambu-Jona-Lasinio models at low N and condensation from pseudogap phase in strong-coupling superconductors. Recent success in describing of Bose-type superconductors that possess two characteristic temperatures and a pseudogap above T, is the develop ment approximately comparable with the BCS theory. One can expect that it should have influence on high-energy physics, similar to impact of the BCS theory on this subject. Although first generalizations of this concept to particle physics were made recently, these results were not systematized. In this review we summarize this development and discuss similarities and differences of the appearance of the pseudogap phase in superconductors and the Gross-Neveu and Nambu-Jona-Lasinio-like models. We discuss its possible relevance for chiral phase transition in QCD and color superconductors. This paper is organized in three parts. In the first part, we briefly review the separation of temperatures of pair formation and pair condensation in strong-coupling and low carrier density superconductors (i.e. the formation of the pseudogap phase). The second part is a review of nonlinear sigma model approach to an analogous phenomenon in the chiral Gross-Neveu model at small N. In the third part we discuss the modified Nambu-Jona-Lasinio model where the chiral phase transition is accompanied by a formation of a phase analogous to the pseudogap phase.
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| 19. |
- Babaev, Egor, et al.
(författare)
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Observability of a projected new state of matter : A metallic superfluid
- 2005
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 95:10
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Tidskriftsartikel (refereegranskat)abstract
- Dissipationless quantum states, such as superconductivity and superfluidity, have attracted interest for almost a century. A variety of systems exhibit these macroscopic quantum phenomena, ranging from superconducting electrons in metals to superfluid liquids, atomic vapors, and even large nuclei. It was recently suggested that liquid metallic hydrogen could form two new and unusual dissipationless quantum states, namely, the metallic superfluid and the superconducting superfluid. Liquid metallic hydrogen is projected to occur only at an extremely high pressure of about 400 GPa, with pressures on hydrogen of 320 GPa having already been reported. The issue to be addressed is whether this state could be experimentally observable in principle. We propose four experimental probes for detecting it.
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| 20. |
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| 21. |
- Babaev, Egor
(författare)
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Phase diagram of planar U(l) x U(l) superconductor - Condensation of vortices with fractional flux and a superfluid state
- 2004
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Ingår i: Nuclear Physics B. - : Elsevier BV. - 0550-3213 .- 1873-1562. ; 686:3, s. 397-412
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Tidskriftsartikel (refereegranskat)abstract
- We discuss a phase diagram of two-dimensional U(1) x U(l) superconductor in the field theoretic formalizm of [Phys. Rev. Lett. 89 (2002) 067001]. In particular we discuss that when penetration length is short the system exhibit a quasi-neutral quasi-superfluid state which is a state when quasi-long range order sets in only in phase difference while individually the phases are disordered.
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| 22. |
- Babaev, Egor, et al.
(författare)
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Rotational response of superconductors : Magnetorotational isomorphism and rotation-induced vortex lattice
- 2014
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 89:10, s. 104501-
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Tidskriftsartikel (refereegranskat)abstract
- The analysis of nonclassical rotational response of superfluids and superconductors was performed by Onsager [Onsager, Nuovo Cimento, Suppl. 6, 279 (1949)] and London [Superfluids (Wiley, NewYork, 1950)] and crucially advanced by Feynman [Prog. Low Temp. Phys. 1, 17 (1955)]. It was established that, in the thermodynamic limit, neutral superfluids rotate by forming-without any threshold-a vortex lattice. In contrast, the rotation of superconductors at angular frequency Omega-supported by uniform magnetic field B-L proportional to Omega due to surface currents-is of the rigid-body type (London law). Here we show that, neglecting the centrifugal effects, the behavior of a rotating superconductor is identical to that of a superconductor placed in a uniform fictitious external magnetic field (H) over tilde = -B-L. In particular, the isomorphism immediately implies the existence of two critical rotational frequencies in type-2 superconductors.
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| 23. |
- Babaev, Egor, et al.
(författare)
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Semi-Meissner state and neither type-I nor type-II superconductivity in multicomponent superconductors
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 72:18
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Tidskriftsartikel (refereegranskat)abstract
- Traditionally, superconductors are categorized as type I or type II. Type-I superconductors support only Meissner and normal states, while type-II superconductors form magnetic vortices in sufficiently strong applied magnetic fields. Recently there has been much interest in superconducting systems with several species of condensates, in fields ranging from condensed matter to high energy physics. Here we show that the classification into types I and II is insufficient for such multicomponent superconductors. We obtain solutions representing thermodynamically stable vortices with properties falling outside the usual type-I/type-II dichotomy, in that they have the following features: (i) Pippard electrodynamics, (ii) interaction potential with long-range attractive and short-range repulsive parts, (iii) for an n-quantum vortex, a nonmonotonic ratio E(n)/n where E(n) is the energy per unit length, (iv) energetic preference for nonaxisymmetric vortex states, vortex molecules. Consequently, these superconductors exhibit an emerging first order transition into a semi-Meissner state, an inhomogeneous state comprising a mixture of domains of two-component Meissner state and vortex clusters.
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| 24. |
- Babaev, Egor
(författare)
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Thermodynamics of the crossover from weak- to strong-coupling superconductivity
- 2001
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 6318:18
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we study an evolution of low-temperature thermodynamical quantities for an electron gas with a delta -function attraction as the system crosses over from weak-coupling (BCS-type) to strong-coupling (Bose-type) superconductivity in three and two dimensions.
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| 25. |
- Babaev, Egor
(författare)
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Topological order in higher composites
- 2024
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Ingår i: Physical Review Research. - : American Physical Society (APS). - 2643-1564. ; 6:3
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Tidskriftsartikel (refereegranskat)abstract
- We introduce the concept of composite topological order in multicomponent systems. In such a state topological order appears only in higher-than-usual composites, with no topological order in elementary fields. We propose that such a state can be realized in Bose-Fermi mixtures in ultracold atoms.
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| 26. |
- Babaev, Egor, et al.
(författare)
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Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors
- 2010
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 105:6, s. 067003-
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Tidskriftsartikel (refereegranskat)abstract
- We show that in multiband superconductors, even an extremely small interband proximity effect can lead to a qualitative change in the interaction potential between superconducting vortices by producing long-range intervortex attraction. This type of vortex interaction results in an unusual response to low magnetic fields leading to phase separation into domains of two-component Meissner states and vortex droplets.
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| 27. |
- Babaev, Egor, et al.
(författare)
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Type-1.5 superconductivity
- 2017
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Ingår i: Superconductors at the Nanoscale: From Basic Research to Applications. - : Walter de Gruyter GmbH. - 9783110456806 - 9783110456202 ; , s. 133-164
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Bokkapitel (övrigt vetenskapligt/konstnärligt)
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| 28. |
- Babaev, Egor, et al.
(författare)
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Type-1.5 Superconductivity in Multiband and Other Multicomponent Systems
- 2013
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Ingår i: Journal of Superconductivity and Novel Magnetism. - : Springer Science and Business Media LLC. - 1557-1939 .- 1557-1947. ; 26:5, s. 2045-2055
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Tidskriftsartikel (refereegranskat)abstract
- Usual superconductors are classified into two categories: of type-1 when the ratio of the magnetic field penetration length (lambda) to coherence length (xi) kappa = lambda/xi < 1/root 2 and of type-2 when kappa > 1/root 2. The boundary case kappa = 1/root 2 is also considered to be a special situation, frequently termed as "Bogomolnyi limit". Here we discuss multicomponent systems which can possess three or more fundamental length scales and allow a separate superconducting state, which was recently termed "type-1.5". In that state, a system has the following hierarchy of coherence and penetration lengths xi(1) < root 2 lambda < xi(2). We also briefly overview the works on single-component regime kappa approximate to 1/root 2 and comment on recent discussion by Brandt and Das in the proceedings of the previous conference in this series.
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| 29. |
- Babaev, Egor, et al.
(författare)
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Type-1.5 superconductivity in multiband systems : Magnetic response, broken symmetries and microscopic theory - A brief overview
- 2012
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Ingår i: Physica. C, Superconductivity. - : Elsevier BV. - 0921-4534 .- 1873-2143. ; 479, s. 2-14
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Tidskriftsartikel (refereegranskat)abstract
- A conventional superconductor is described by a single complex order parameter field which has two fundamental length scales, the magnetic field penetration depth lambda and the coherence length xi. Their ratio kappa determines the response of a superconductor to an external field, sorting them into two categories as follows; type-I when kappa < 1/root 2 and type-II when kappa > 1/root 2. We overview here multicomponent systems which can possess three or more fundamental length scales and allow a separate "type-1.5" superconducting state when, e. g. in two-component case xi(1) < root 2 lambda < xi(2). In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges. As a consequence the system should form an additional Semi-Meissner state which properties we discuss below. In that state vortices form clusters in low magnetic fields. Inside the cluster one of the component is depleted and the superconductor-to-normal interface has negative energy. In contrast the current in second component is mostly concentrated on the cluster's boundary, making the energy of this interface positive. Here we briefly overview recent developments in Ginzburg-Landau and microscopic descriptions of this state.
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| 30. |
- Babaev, Egor, et al.
(författare)
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Type-1.5 superconductivity in multicomponent systems
- 2017
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Ingår i: Physica. C, Superconductivity. - : ELSEVIER SCIENCE BV. - 0921-4534 .- 1873-2143. ; 533, s. 20-35
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Tidskriftsartikel (refereegranskat)abstract
- In general a superconducting state breaks multiple symmetries and, therefore, is characterized by several different coherence lengths i = 1,..., N. Moreover in multiband material even superconducting states that break only a single symmetry are nonetheless described, under certain conditions by multi component theories with multiple coherence lengths. As a result of that there can appear a state where some coherence lengths are smaller and some are larger than the magnetic field penetration length A: xi(1) <= xi(2)...
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| 31. |
- Babaev, Egor, et al.
(författare)
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Type-1.5 superconductivity in two-band systems
- 2010
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Ingår i: Physica. C, Superconductivity. - : Elsevier BV. - 0921-4534 .- 1873-2143. ; 470:19, s. 717-721
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Tidskriftsartikel (refereegranskat)abstract
- In the usual Ginzburg-Landau theory the critical value of Ginzburg-Landau parameter kappa(c) = 1/root 2 separates regimes of type-I and type-II superconductivity. The latter regime possess thermodynamically stable vortex excitations which interact with each other repulsively and tend to form vortex lattices. It was shown in [5] that this dichotomy in broken in U(1) x U(1) Ginzburg-Landau models which possess a distinct phase with vortex excitations which interact attractively at large length scales and repulsively at shorter distances. Here we discuss the influence of the Josephson coupling and that similar kind of superconductivity can also arise for entirely different reasons in superconductors where only one band is superconducting if this band interacting via a proximity effect with another band (the report is partially based on [1]).
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| 32. |
- Babaev, Egor
(författare)
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Unconventional Rotational Responses of Hadronic Superfluids in a Neutron Star Caused by Strong Entrainment and a Sigma(-) Hyperon Gap
- 2009
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 103:23
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Tidskriftsartikel (refereegranskat)abstract
- I show that the usual model of the rotational response of a neutron star, which predicts rotation-induced neutronic vortices and no rotation-induced protonic vortices, does not hold (i) beyond a certain threshold of entrainment interaction strength nor (ii) in the case of nonzero Sigma(-) hyperon gap. I show that in both of these cases the rotational response involves the creation of phase windings in an electrically charged condensate. Lattices of bound states of vortices which result from these phase windings can (for a range of parameters) strongly reduce the interaction between rotation-induced vortices with magnetic-field carrying superconducting components.
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| 33. |
- Babaev, Egor, et al.
(författare)
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Violation of the London law and Onsager-Feynman quantization in multicomponent superconductors
- 2007
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Ingår i: Nature Physics. - : Springer Science and Business Media LLC. - 1745-2473 .- 1745-2481. ; 3:8, s. 530-533
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Tidskriftsartikel (refereegranskat)abstract
- Non-classical response to rotation is a hallmark of quantum ordered states such as superconductors and superfluids. The rotational responses of all currently known single-component 'super' states of matter (superconductors, superfluids and supersolids) are largely described by two fundamental principles and fall into two categories according to whether the systems are composed of charged or neutral particles: the London law relating the angular velocity to a subsequently established magnetic field and the Onsager-Feynman quantization of superfluid velocity. These laws are theoretically shown to be violated in a two-component superconductor such as the projected liquid metallic states of hydrogen and deuterium at high pressures. The rotational responses of liquid metallic hydrogen or deuterium identify them as a new class of dissipationless states; they also directly point to a particular experimental route for verification of their existence.
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| 34. |
- Babaev, Egor
(författare)
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Vortex matter, effective magnetic charges, and generalizations of the dipolar superfluidity concept in layered systems
- 2008
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 77:5
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Tidskriftsartikel (refereegranskat)abstract
- In the first part of this paper, we discuss electrodynamics of an excitonic condensate in a bilayer. We show that under certain conditions, the system has a dominant energy scale and is described by the effective electrodynamics with "planar magnetic charges." In the second part of the paper, we point out that a vortex liquid state in bilayer superconductors also possesses dipolar superfluid modes and establish equivalence mapping between this state and a dipolar excitonic condensate. We point out that a vortex liquid state in a N-layer superconductor possesses multiple topologically coupled dipolar superfluid modes and therefore represents a generalization of the dipolar superfluidity concept.
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| 35. |
- Babaev, Egor
(författare)
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Vortices carrying an arbitrary fraction of magnetic flux quantum, neutral superfluidity and knotted solitons in two-gap Ginzburg-Landau model
- 2004
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Ingår i: Physica. C, Superconductivity. - : Elsevier BV. - 0921-4534 .- 1873-2143. ; 404:04-jan, s. 39-43
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Tidskriftsartikel (refereegranskat)abstract
- We show that in two-gap superconductors there exist vortices which carry an arbitrary fraction of magnetic flux quantum and in two dimensions under certain conditions these vortices undergo a BKT transition which marks onset of quasi-long-range order only in a difference of phases of the two order parameters. In the second part of the talk we show that an U(1) x U(1) Ginzburg-Landau model or a GL model where U(1) x U(1) symmetry is weakly broken to U(1) is exactly equivalent to a version of the Faddeev's the nonlinear O(3) sigma model. This implies in particular that such a system possesses a hidden O(3) symmetry and besides that allows for the formation of knotted solitons. The second part of the talk is based on a joint work with L.D. Faddeev and A. Niem: Phys. Rev. B 65 (2002) 100512.
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| 36. |
- Babaev, Egor
(författare)
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Vortices with fractional flux in two-gap superconductors and in extended Faddeev model
- 2002
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 89, s. 067001-
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Tidskriftsartikel (refereegranskat)abstract
- We discuss linear topological defects allowed in two-gap superconductors and equivalent extended Faddeev model. We show that, in these systems, there exist vortices which carry an arbitrary fraction of magnetic flux quantum. Besides that, we discuss topological defects which do not carry magnetic flux and describe features of ordinary one-magnetic-flux-quantum vortices in the two-gap system. The results could be relevant for the newly discovered two-band superconductor MgB2.
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| 37. |
- Barkman, Mats, et al.
(författare)
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Antichiral and nematicity-wave superconductivity
- 2019
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 99:22
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Tidskriftsartikel (refereegranskat)abstract
- Larkin-Ovchinnikov superconducting state has spontaneous modulation of Cooper pair density, while Fulde-Ferrell state has a spontaneous modulation in the phase of the order parameter. We report that a quasi-two-dimensional Dirac metal, under certain conditions has principally different inhomogeneous superconducting states that by contrast have spontaneous modulation in a submanifold of a multiple-symmetries-breaking order parameter. The first state we find can be viewed as a nematic superconductor where the nematicity vector spontaneously breaks rotational and translational symmetries due to spatial modulation. The other demonstrated state is a chiral superconductor with spontaneously broken time-reversal and translational symmetries. It is characterized by an order parameter, which forms a lattice pattern of alternating chiralities.
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| 38. |
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| 39. |
- Barkman, Mats, et al.
(författare)
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Elevated critical temperature at BCS superconductor-band insulator interfaces
- 2022
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 105:22
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Tidskriftsartikel (refereegranskat)abstract
- We consider the interface between a Bardeen-Cooper-Schrieffer superconductor and nonsuperconducting band insulator. We show that under certain conditions, such interfaces can have an elevated superconducting critical temperature, without increasing the strength of the pairing interaction at the interface. We identify the regimes where the interface critical temperature exceeds the critical temperature associated with a superconductor vacuum interface.
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| 40. |
- Barkman, Mats, et al.
(författare)
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Ring solitons and soliton sacks in imbalanced fermionic systems
- 2020
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Ingår i: Physical Review Research. - : American Physical Society. - 2643-1564. ; 2:4
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Tidskriftsartikel (refereegranskat)abstract
- We show that in superfluids with fermionic imbalance and uniform ground state, there are stable solitons. These solutions are formed of radial density modulations resulting in nodal rings. We demonstrate that these solitons exhibit nontrivial soliton-soliton and soliton-vortex interactions and can form complicated bound states in the form of "soliton sacks." In a phase-modulating (Fulde-Ferrell) background, we find different solitonic states, in the form of stable vortex-antivortex pairs.
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| 41. |
- Barkman, Mats
(författare)
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Superconducting surfaces, solitons and skyrmions
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis focuses on superconductivity, a field within condensed matter physics which since its experimental discovery roughly a century ago, not only has lead to significant contributions revealing the fundamental theories of physics, but also to practical applications. This includes for example quantum vortices, which play paramount roles both in other condensed matter settings, but also in high-energy physics. The dissipationless currents in superconductors are essential to achieve the strong magnetic fields necessary when performing Magnetic Resonant Imaging (MRI). My research on superconductors spans across three topics: superconducting surfaces, multiband superconductivity and inhomogeneous states formed in imbalanced superfluids. A brief introduction and summary of the scientific contribution of this thesis to each of these topics is given below. Bardeen-Cooper-Schrieffer (BCS) theory tells us that from a microscopic perspective, superconductivity is the phenomenon of condensation of bound electron pairs, so-called Cooper pairs. The superconducting state is described by a complex-valued field known as the superconducting gap parameter. In the most simple superconducting materials, where there is only one electronic band, only one complex field is necessary to describe the superconducting state, which spontaneously breaks U(1)-symmetry. In other superconducting materials, such as the iron-based superconductor Ba1−xKxFe2As2, the band structure is more complicated and multiple electronic bands are present. Such multiband superconductors may require multiple complex fields to describe the superconducting state, which can spontaneously break other symmetries, such as time-reversal symmetry, in addition to U(1)-symmetry. Two proposed pairing symmetries for spontaneous time-reversal symmetry breaking (TRSB) spin-singlet superconductors are s+is and s+id. In Paper IV, we demonstrate how magnetic features of pinned domain walls in anisotropic TRSB superconductors can be used to distinguish between s+is and s+id pairing. Classifying topological excitations in superconductors is crucial to understand the superconducting state. For example, quantum vortices are key in understanding the magnetic response of type-II superconductors, and the thermal fluctuations-induced phase transitions in superconductors and superfluids. It has been hypothesized that multiband superconductors, which are described by multiple complex fields, can host topological excitations which are different from the ordinary quantum vortices. Understanding the properties of these new topological excitations carries similar importance to that of ordinary quantum vortices. In Paper VII and Paper VIII, we provide the first microscopic demonstration of multiband fractional vortices and CP2-skyrmions using fully self-consistent Bogoliubov-de Gennes (BdG) theory. Previous demonstrations of such topological excitations have been done using classical field theory approaches, such as Ginzburg-Landau (GL) theory. Our BdG calculations maintain microscopic degrees of freedom which are neglected using GL and quasiclassical theories of superconductivity. The most well-known inhomogeneous superconducting phase is the Abrikosov vortex lattice, which forms in the presence of an external magnetic field in type-II superconductors. Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed another type of inhomogeneous superconducting state, which may form in the presence of a sufficiently large population imbalance between spin up and spin down electrons. The origin of this supersolid state is the formation of Cooper pairs with non-zero net momentum due to spin-dependent Fermi surfaces. In Paper V, we show that spin-imbalanced superfluids can host a unique type of solitons, even before the FFLO regime is entered. These solitons are not present in ordinary uniform superconducting states, and can therefore act as identifiable traces of the FFLO state. The Fulde-Ferrell state and the Larkin-Ovchinnikov state are characterized respectively by modulation in the phase and the density of the superconducting gap parameter. In Paper II, we explored the possibility of other types of inhomogeneous states caused by imbalance in multiband superconductors. Using GL theory, we demonstrated two new types of inhomogeneous states, characterized by spatially alternating chirality and nematicity. Understanding the superconducting properties of surfaces and boundaries is important, both fundamentally to the theory of superconductivity and practically in the construction of superconducting devices. In Paper I and Paper III we demonstrate using both GL and BdG theory that pair-density-wave superconductors support superconducting surface states with critical temperatures larger than the bulk critical temperature. In Paper VI we show increased critical temperatures of superconductor-insulator interfaces. The increase in critical temperature occurs without locally increasing the superconducting pairing strength near the boundaries, or without the introduction of modified surface phonons.
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| 42. |
- Barkman, Mats, et al.
(författare)
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Surface Pair-Density-Wave Superconducting and Superfluid States
- 2019
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 122:16
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Tidskriftsartikel (refereegranskat)abstract
- Fulde, Ferrell, Larkin, and Ovchinnikov (FFLO) predicted inhomogeneous superconducting and superfluid ground states, spontaneously breaking translation symmetries. In this Letter, we demonstrate that the transition from the FFLO to the normal state as a function of temperature or increased Fermi surface splitting is not a direct one. Instead, the system has an additional phase transition to a different state where pair-density-wave superconductivity (or superfluidity) exists only on the boundaries of the system, while the bulk of the system is normal. The surface pair-density-wave state is very robust and exists for much larger fields and temperatures than the FFLO state.
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| 43. |
- Benfenati, Andrea, et al.
(författare)
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Boundary effects in two-band superconductors
- 2021
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 103:14
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Tidskriftsartikel (refereegranskat)abstract
- We present a microscopic study of the behavior of the order parameters near the boundaries of a two-band superconducting material, described by the standard tight-binding Bardeen-Cooper-Schrieffer model. We find superconducting surface states. The relative difference between bulk and surface critical temperatures is a nontrivial function of the interband coupling strength. For superconductors with weak interband coupling, boundaries induce variations of the gaps with the presence of multiple length scales, despite nonzero interband Josephson coupling.
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| 44. |
- Benfenati, Andrea, et al.
(författare)
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Demonstration of CP2 skyrmions in three-band superconductors by self-consistent solutions of a Bogoliubov-de Gennes model
- 2023
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 107:9
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Tidskriftsartikel (refereegranskat)abstract
- Topological defects, such as magnetic-flux-carrying quantum vortices, determine the magnetic response of superconductors and hence are of fundamental importance. Here, we show that stable CP2 skyrmions exist in three-band s + is superconductors as fully self-consistent solutions to a microscopic Bogoliubov-de Gennes model. This allows us to calculate microscopically the magnetic signatures of CP2 skyrmions and their footprint in the local density of states.
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| 45. |
- Benfenati, Andrea, et al.
(författare)
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Demonstration of CP2 skyrmions in three-band superconductors by self-consistent solutions to a Bogoliubov-de Gennes model
- 2022
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Topological defects, such as magnetic-flux-carrying quantum vortices determine the magnetic response of superconductors and hence are of fundamental importance. Here, we show that stable CP2 skyrmions exist in three-band s+is superconductors as fully self-consistent solutions to a microscopic Bogoluibov-de Gennes model. This allows us to calculate microscopically the magnetic signatures of CP2 skyrmions and their footprint in the local density of states.
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| 46. |
- Benfenati, Andrea Ludovico, 1994-
(författare)
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Numerical solutions to non-linear inhomogeneous problems in Superconductivity : From sphalerons to multi-band boundary states and spontaneous magnetic fields
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is a compilation of theoretical works focused on simulating and studying open questions regarding single and multiband superconductivity. In the last decades, with the discovery of multiband superconductors, the spectrum of potential applications has greatly widened. Superconductors are not only employed to realize dissipationless current carrying devices, but are used to construct quantum-based measurement instruments, such as single photon detectors as well as superconducting qubits. The properties of superconductors, as critical temperatures and vortex nucleation barriers are of key importance for applications, and still poorly understood. They are strongly affected by the physics of the boundaries, as well as by the sample's geometry and by the presence of impurities. The open questions can be answered with new theoretical methods, which can then guide and optimize the construction process of superconducting devices, which constitutes a crucial challenge today. There are several models that can be utilized to describe superconductors, from the microscopic Bardeen Cooper Schrieffer theory, up to the macroscopic Ginzburg Landau models. Each of these theories carries advantages and limitations, making it impossible to rely only on a specific model. In this thesis we utilize microscopic and macroscopic models to answer the following questions:How can we determine the free energy barriers to vortex nucleation in single band and multiband superconductors without relying on uncontrolled approximations?What are the properties of the superconducting states which spontaneously break time reversal symmetry?How do boundaries and interfaces influence the critical temperatures of superconductors?We answer these questions in eight papers, which we shortly summarize in the following. In Paper 1, Magnetic signatures of domain walls in s+is and s+id superconductors: Observability and what that can tell us about the superconducting order parameter, we consider an effective two-band anisotropic Ginzburg Landau model, describing a superconductor breaking time reversal symmetry. There is high interest on spontaneous time reversal symmetry breaking due to recent muon-spin rotation experiments, claiming to measure spontaneous magnetic field in Fe-based superconductors such as Ba1-xKxFe2As2. However, the symmetry of the superconducting order parameters remains undetermined, and the most promising candidates are s+is and s+id states. In the work, we obtain solutions for domain walls within the Ginzburg Landau model. By studying the spontaneous magnetic signatures of domain walls, having different orientations with respect to the crystalline axes, for both s+is and s+id order parameters, we demonstrate their differences and propose a procedure to infer the order parameter's symmetry from magnetic field measurements.In Paper 2, Vortex nucleation barrier in superconductors beyond the Bean-Livingston approximation: A numerical approach for the sphaleron problem in a gauge theory, we address the long standing problem of calculating the energy barriers for the vortex nucleation in a superconductor. The only available tool to do so, was the Bean-Livingston theory, which relies on uncontrollable approximations. This does not allow to take into account the non-linear nature of the Ginzburg Landau model, or the presence of impurities and surface roughness. Therefore, we develop the gauged string method, a gauge invariant numerical framework, based on the simplified string method, which enables us to accurately compute the minimum free energy path for the vortex nucleation. Moreover, we present a study of how the nucleation energy barrier changes in the presence of impurities and surface roughness. In Paper 3, Vortex nucleation barriers and stable fractional vortices near boundaries in multicomponent superconductors, we extend the gauged string method to multiband superconductors, where the energy landscape is much broader than in the single band case, and the number of possible processes is higher. In multiband superconductors the topological excitations are fractional vortices, which once bounded, form composite vortices. Fractional vortices are energetically unfavorable, as they are associated to an energy cost which scales logarithmically with the system size. Once they bind and form a composite vortex, the extra energy cost is canceled. However, it was previously shown in the London model that fractional vortices can be stabilized near boundaries. In this paper, we study the energy barriers for the nucleation of fractional vortices, and for the formation composite vortices. Moreover, we show how the presence of anisotropies can influence such barriers. Then we study how the same processes are influenced by the interband Josephson interactions. By using the gauged string method, we demonstrate how the fractionalized nucleation process results in multiple saddle points and intermediate metastable configurations.In Paper 4, Boundary effects in two-band superconductors, we study microscopically the behavior of the superconducting order parameters near the boundaries of a two-band s-wave superconductor. We describe the system using a tight binding Bardeen Cooper Schrieffer model with interband interaction. We show the existence of surface states, and calculate how the difference between bulk and surface critical temperatures depends on the strength of the interband coupling. Then, we focus the analysis on weak interband interactions to show, at the level of a fully microscopic theory, how the variations of the gaps near the boundaries occur with multiple length scales. In Paper 5, Spontaneous edge and corner currents in s+is superconductors and time-reversal-symmetry-breaking surface states, we consider a three band superconductor, described with a microscopic tight binding Bardeen Cooper Schrieffer model with interband interaction. In the current classification scheme, an s+is state is a non-topological and non-chiral state, which does not exhibit topological surface states and therefore no spontaneous surface currents. In the article, we consider a system where the three bands have slightly different intraband pairing potential but the same interband coupling, resulting in slightly asymmetric bands. We show that, as the temperature is increased, the state which spontaneously break time reversal symmetry becomes localized near the sample boundaries, and generate spontaneous magnetic signatures. Finally, we show how, by changing the sample geometry, the magnetic signatures can be enhanced. We underline that, this phenomenon is not a general property of time reversal symmetry breaking states, but can account for the presence of spontaneous magnetic fields in s+is superconductors and cannot be predicted using the macroscopic Ginzburg Landau theory. Moreover, the paper shows that spontaneous surface currents can arise for non-topological reasons.In Paper 6, Demonstration of CP2 skyrmions in three-band superconductors by self-consistent solutions to a Bogoliubov-de-Gennes model, we continue the study of three component s+is superconductors, described using a microscopic tight binding Bardeen Cooper Schrieffer model. In this work, we consider three symmetric bands, and we study the CP2 skyrmionic topological excitations of the system. We present not only the configurations of the superconducting order parameters, but also the respective magnetic field and density of states. Moreover, the simulation of CP2 skyrmions in superconductors, described a with fully microscopic model, had not been done before. In the context of superconductivity, CP2 skyrmion solutions were previously described only within the phenomenological macroscopic Ginzburg-Landau theory.In Paper 7, Pair-density-wave superconductivity of faces, edges, and vertices in systems with imbalanced fermions we analyze the boundary effects in superconductors exhibiting Fulde-Ferrell-Larkin-Ovchinnikov states. We do so by employing and comparing Bogoliubov-de-Gennes and Ginzburg Landau formalisms. We show that, within the Ginzburg Landau theory, in a three dimensional superconductor, there is a sequence of phase transitions as the temperature increases. Then, we perform the same sequence of simulations for two dimensional samples described using the Bogoliubov-de-Gennes formalism, showing the same sequence of phase transitions.In Paper 8, Elevated critical temperature at BCS superconductor-band insulator interfaces, we study the physics of interfaces between a superconductor, described using a tight-binding mean field Hamiltonian, and a band insulator. We limit the study to one-dimensional samples and demonstrate that, within certain parameter ranges, it is indeed possible to enhance the critical temperature in the vicinity of the interface. This occurs without changing the strength of the superconducting-pairing interaction. Then we present the parameters regimes in which the near-interface critical temperature exceeds the critical temperature of a conventional superconductor-vacuum interface.
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| 47. |
- Benfenati, Andrea, et al.
(författare)
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Magnetic signatures of domain walls in s plus is and s plus id superconductors : Observability and what that can tell us about the superconducting order parameter
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:5
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Tidskriftsartikel (refereegranskat)abstract
- One of the defining features of spontaneously broken time-reversal symmetry (BTRS) is the existence of domain walls, the detection of which would be strong evidence for such systems. There is keen interest in BTRS currently, in part, due to recent muon spin rotation experiments, which have pointed towards Ba1-xKxFe2As2 exhibiting a remarkable case of s-wave superconductivity with spontaneously broken time-reversal symmetry. A key question, however, is how to differentiate between the different theoretical models which describe such a state. Two particularly popular choices of model are s + is and s + id superconducting states. In this paper, we obtain solutions for domain walls in s + is and s + id systems, including the effects of lattice anisotropies. We show that, in general, both models exhibit spontaneous magnetic fields that extend along the entire length of the domain wall. We demonstrate the qualitative difference between the magnetic signatures of s + is and s + id domain walls and propose a procedure to extract the superconducting pairing symmetry from the magnetic-field response of domain walls.
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| 48. |
- Benfenati, Andrea, et al.
(författare)
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Spontaneous edge and corner currents in s+is superconductors and time reversal symmetry breaking surface states
- 2022
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 105:13, s. 134518-
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Tidskriftsartikel (refereegranskat)abstract
- We present a study of the basic microscopic model of a s-wave superconductor with frustrated interbandinteraction. When frustration is strong, such an interaction gives raise to a s + is state. This is a s-wave superconductor that spontaneously breaks time reversal symmetry. We show that in addition to the known s + is state,there is additional phase where the system’s bulk is a conventional s-wave state, but superconducting surfacestates break time reversal symmetry. Furthermore, we show that s + is superconductors can have spontaneousboundary currents and spontaneous magnetic fields. These arise at lower-dimensional boundaries, namely, thecorners in two-dimensional samples. This demonstrates that boundary currents effects in superconductors canarise in states which are not topological and not chiral according to the modern classification.
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| 49. |
- Benfenati, Andrea, et al.
(författare)
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Vortex nucleation barrier in superconductors beyond the Bean-Livingston approximation : A numerical approach for the sphaleron problem in a gauge theory
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 101:22
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Tidskriftsartikel (refereegranskat)abstract
- The knowledge of vortex nucleation barriers is crucial for applications of superconductors, such as single-photon detectors and superconductor-based qubits. Contrarily to the problem of finding energy minima and critical fields, there are no controllable methods to explore the energy landscape, identify saddle points, and compute associated barriers. Similar problems exist in high-energy physics where the saddle-point configurations are called sphalerons. Here, we present a generalization of the string method to gauge field theories, which allows the calculation of energy barriers in superconductors. We solve the problem of vortex nucleation, assessing the effects of the nonlinearity of the model, complicated geometry, surface roughness, and pinning.
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| 50. |
- Biswas, P. K., et al.
(författare)
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Coexistence of type-I and type-II superconductivity signatures in ZrB12 probed by muon spin rotation measurements
- 2020
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 102:14
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Tidskriftsartikel (refereegranskat)abstract
- Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example, at different temperatures, is rare in nature. We employed the muon spin rotation (mu SR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most important, we found a region showing unusual behavior where the usually mutually exclusive mu SR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modeling that required taking into account multiple bands and multiple coherence lengths, which suggests that material has one coherence length larger and another smaller than the magnetic field penetration length (the type-1.5 regime). At stronger fields, a footprint of the type-II mixed state showing square flux-line lattice was also obtained using neutron diffraction.
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