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Multicomponent superconductivity: Vortex matter and phase transitions
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The topic of this thesis is vortex-physics in multi component Ginzburg- Landau models. These models describe a newly discovered class of super- conductors with multiple superconducting gaps, and possess many properties that set them apart from single component models. The work presented here relies on large scale computer simulations using various numerical techniques, but also on some analytical methods.

In Paper I, Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors, we show that in multiband supercon- ductors, even an extremely small interband proximity effect can lead to a qualitative change in the interaction potential between superconducting vor- tices, 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.

In paper II, Type-1.5 superconductivity in two-band systems, we discuss the influence of Josephson coupling and show that non-monotonic intervortex interaction can also arise in two-band superconductors where one of the bands is proximity induced by Josephson interband coupling.

In paper III, Type-1.5 superconductivity in multiband systems: Effects of interband couplings, we investigate the appearance of Type-1.5 superconduc- tivity in the case with two active bands and substantial inter-band couplings such as intrinsic Josephson coupling, mixed gradient coupling, and density- density interactions. We show that in the presence of these interactions, the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by linear combinations of the density fields.

In paper IV, Semi-Meissner state and nonpairwise intervortex interactions in type-1.5 superconductors, we demonstrate the existence of nonpairwise in- tervortex forces in multicomponent and layered superconducting systems. We also consider the properties of vortex clusters in a semi-Meissner state of type- 1.5 two-component superconductors. We show that under certain conditions nonpairwise forces can contribute to the formation of complex vortex states in type-1.5 regimes.

In paper V, Length scales, collective modes, and type-1.5 regimes in three- band superconductors, we consider systems where frustration in phase dif- ferences occur due to competing Josephson inter-band coupling terms. We show that gradients of densities and phase differences can be inextricably intertwined in vortex excitations in three-band models. This can lead to long-range attractive intervortex interactions and the appearance of type-1.5 regimes even when the intercomponent Josephson coupling is large. We also show that field-induced vortices can lead to a change of broken symmetry from U (1) to U (1) ⇥ Z2 in the system. In the type-1.5 regime, it results in a semi-Meissner state where the system has a macroscopic phase separation in domainswithbrokenU(1)andU(1)⇥Z2 symmetries.

In paper VI, Topological Solitons in Three-Band Superconductors with Broken Time Reversal Symmetry, we show that three-band superconductors with broken time reversal symmetry allow magnetic flux-carrying stable topo- logical solitons. They can be induced by fluctuations or quenching the system through a phase transition. It can provide an experimental signature of the time reversal symmetry breakdown.

In paper VII, Type-1.5 superconductivity in multiband systems: Magnetic response, broken symmetries and microscopic theory – A brief overview, we give an overview of vortex physics and magnetic response in multi component Ginzburg-Landau theory. We also examine Type-1.5 superconductivity in the context of microscopic theory.

In paper VIII, Chiral CP2 skyrmions in three-band superconductors, we show that under certain conditions, three-component superconductors (and, in particular, three-band systems) allow stable topological defects different from vortices. We demonstrate the existence of these excitations, charac- terised by a CP2 topological invariant, in models for three-component super- conductors with broken time-reversal symmetry. We term these topological defects “chiral GL(3) skyrmions,” where “chiral” refers to the fact that due to broken time-reversal symmetry, these defects come in inequivalent left- and right-handed versions. In certain cases, these objects are energetically cheaper than vortices and should be induced by an applied magnetic field. In other situations, these skyrmions are metastable states, which can be produced by a quench. Observation of these defects can signal broken time-reversal sym- metry in three-band superconductors or in Josephson-coupled bilayers of s± and s-wave superconductors.

In paper IX, Phase transition in multi-component superconductors, we ex- amine the thermodynamics of frustrated multi-components superconductors and show that their highly complex energy landscape can give rise new types of phase transitions not present in single component superconductors. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , 57 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2013:62
Keyword [en]
Superconductivity, Ginzburg Landau, field theory, Iron pnictide
National Category
Natural Sciences Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-136279ISBN: 978-91-7501-924-6 (print)OAI: oai:DiVA.org:kth-136279DiVA: diva2:675699
Public defence
2013-12-19, FR4, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131205

Available from: 2013-12-05 Created: 2013-12-04 Last updated: 2014-02-14Bibliographically approved
List of papers
1. Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors
Open this publication in new window or tab >>Type-1.5 Superconducting State from an Intrinsic Proximity Effect in Two-Band Superconductors
2010 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 105, no 6, 067003- p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Interaction potentials, Interband, Low magnetic fields, Meissner state, Multiband superconductors, Proximity effects, Qualitative changes, Superconducting state, Superconducting vortices, Two-band superconductors, Two-component, Vortex interactions, Magnetic domains, Magnetic fields, Phase separation, Superconducting materials, Vortex flow
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-29414 (URN)10.1103/PhysRevLett.105.067003 (DOI)000280612400010 ()2-s2.0-77955320910 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20110216Available from: 2011-02-16 Created: 2011-02-02 Last updated: 2017-12-11Bibliographically approved
2. Type-1.5 superconductivity in two-band systems
Open this publication in new window or tab >>Type-1.5 superconductivity in two-band systems
2010 (English)In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 470, no 19, 717-721 p.Article in journal (Refereed) Published
Abstract [en]

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]).

Keyword
Vortices, Type-1.5 superconductivity, Vortex clusters, Vortex interaction, Semi-Meissner state
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-29207 (URN)10.1016/j.physc.2010.02.025 (DOI)000282454400002 ()2-s2.0-77956171082 (Scopus ID)
Note
QC 20110131Available from: 2011-01-31 Created: 2011-01-27 Last updated: 2017-12-11Bibliographically approved
3. Type-1.5 superconductivity in multiband systems: Effects of interband couplings
Open this publication in new window or tab >>Type-1.5 superconductivity in multiband systems: Effects of interband couplings
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 17, 174509- p.Article in journal (Refereed) Published
Abstract [en]

In contrast to single-component superconductors, which are described at the level of Ginzburg-Landau theory by a single parameter kappa and are divided in type-I kappa < 1/root 2 and type-II kappa > 1/root 2 classes, two-component systems in general possess three fundamental length scales and have been shown to possess a separate "type-1.5" superconducting state. In that state, as a consequence of the extra fundamental length scale, vortices attract one another at long range but repel at shorter ranges, and therefore should form clusters in low magnetic fields. In this work we investigate the appearance of type-1.5 superconductivity and the interpretation of the fundamental length scales in the case of two active bands with substantial interband couplings such as intrinsic Josephson coupling, mixed gradient coupling, and density-density interactions. We show that in the presence of substantial intercomponent interactions of the above types the system supports type-1.5 superconductivity with fundamental length scales being associated with the mass of the gauge field and two masses of normal modes represented by mixed combinations of the density fields.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-34200 (URN)10.1103/PhysRevB.83.174509 (DOI)000290477200010 ()2-s2.0-79961122308 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note
QC 20110610Available from: 2011-06-10 Created: 2011-05-30 Last updated: 2017-12-11Bibliographically approved
4. Semi-Meissner state and nonpairwise intervortex interactions in type-1.5 superconductors
Open this publication in new window or tab >>Semi-Meissner state and nonpairwise intervortex interactions in type-1.5 superconductors
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 13, 134515- p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate the existence of nonpairwise interaction forces between vortices in multicomponent and layered superconducting systems. That is, in contrast to most common models, the interaction in a group of such vortices is not a universal superposition of Coulomb or Yukawa forces. Next, we consider the properties of vortex clusters in a semi-Meissner state of type-1.5 two-component superconductors. We show that under certain conditions nonpairwise forces can contribute to the formation of very complex vortex states in type-1.5 regimes.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-45586 (URN)10.1103/PhysRevB.84.134515 (DOI)000295868900009 ()2-s2.0-80155143574 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20111107Available from: 2011-11-07 Created: 2011-10-31 Last updated: 2017-12-08Bibliographically approved
5. Length scales, collective modes, and type-1.5 regimes in three-band superconductors
Open this publication in new window or tab >>Length scales, collective modes, and type-1.5 regimes in three-band superconductors
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 13, 134518- p.Article in journal (Refereed) Published
Abstract [en]

The recent discovery of iron pnictide superconductors has resulted in a rapidly growing interest in multiband models with more than two bands. In this work we specifically focus on the properties of three-band Ginzburg-Landau models which do not have direct counterparts in more studied two-band models. First we derive normal modes and characteristic length scales in the conventional U(1) three-band Ginzburg-Landau model as well as in its time-reversal symmetry-broken counterpart with U(1) x Z(2) symmetry. We show that, in the latter case, the normal modes are mixed phase-density collective excitations. A possibility of the appearance of a massless mode associated with fluctuations of the phase difference is also discussed. Next we show that gradients of densities and phase differences can be inextricably intertwined in vortex excitations in three-band models. This can lead to very long-range attractive intervortex interactions and the appearance of type-1.5 regimes even when the intercomponent Josephson coupling is large. In some cases it also results in the formation of a domainlike structure in the form of a ring of suppressed density around a vortex across which one of the phases shifts by p. We also show that field-induced vortices can lead to a change of broken symmetry from U(1) to U(1) x Z(2) in the system. In the type-1.5 regime, it results in a semi-Meissner state where the system has a macroscopic phase separation in domains with broken U(1) and U(1) x Z(2) symmetries.

Keyword
2-BAND SUPERCONDUCTORS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-47977 (URN)10.1103/PhysRevB.84.134518 (DOI)000296287500007 ()2-s2.0-80155184781 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note
QC 20111117Available from: 2011-11-17 Created: 2011-11-15 Last updated: 2017-12-08Bibliographically approved
6. Topological Solitons in Three-Band Superconductors with Broken Time Reversal Symmetry
Open this publication in new window or tab >>Topological Solitons in Three-Band Superconductors with Broken Time Reversal Symmetry
2011 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 107, no 19, 197001- p.Article in journal (Refereed) Published
Abstract [en]

We show that three-band superconductors with broken time reversal symmetry allow magnetic flux-carrying stable topological solitons. They can be induced by fluctuations or quenching the system through a phase transition. It can provide an experimental signature of the time reversal symmetry breakdown.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-50313 (URN)10.1103/PhysRevLett.107.197001 (DOI)000297006100008 ()2-s2.0-80155137550 (Scopus ID)
Note
QC 20120119Available from: 2012-01-19 Created: 2011-12-05 Last updated: 2017-12-08Bibliographically approved
7. Type-1.5 superconductivity in multiband systems: Magnetic response, broken symmetries and microscopic theory - A brief overview
Open this publication in new window or tab >>Type-1.5 superconductivity in multiband systems: Magnetic response, broken symmetries and microscopic theory - A brief overview
Show others...
2012 (English)In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 479, 2-14 p.Article in journal (Refereed) Published
Abstract [en]

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.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-103378 (URN)10.1016/j.physc.2012.01.002 (DOI)000308580600002 ()2-s2.0-84865738643 (Scopus ID)
Funder
Knut and Alice Wallenberg FoundationSwedish Research Council
Note

QC 20121016

Available from: 2012-10-16 Created: 2012-10-11 Last updated: 2017-12-07Bibliographically approved
8. Chiral CP2 skyrmions in three-band superconductors
Open this publication in new window or tab >>Chiral CP2 skyrmions in three-band superconductors
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 1, 014507- p.Article in journal (Refereed) Published
Abstract [en]

It is shown that under certain conditions, three-component superconductors (and, in particular, three-band systems) allow stable topological defects different from vortices. We demonstrate the existence of these excitations, characterized by a CP2 topological invariant, in models for three-component superconductors with broken time-reversal symmetry. We term these topological defects "chiral GL((3)) skyrmions," where "chiral" refers to the fact that due to broken time-reversal symmetry, these defects come in inequivalent left-and right-handed versions. In certain cases, these objects are energetically cheaper than vortices and should be induced by an applied magnetic field. In other situations, these skyrmions are metastable states, which can be produced by a quench. Observation of these defects can signal broken time-reversal symmetry in three-band superconductors or in Josephson-coupled bilayers of s(+/-) and s-wave superconductors.

Keyword
Strings, Model, Equations
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-118614 (URN)10.1103/PhysRevB.87.014507 (DOI)000313423300004 ()2-s2.0-84872934015 (Scopus ID)
Funder
Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20130222

Available from: 2013-02-22 Created: 2013-02-21 Last updated: 2017-12-06Bibliographically approved

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