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Phase transitions in novel superfluids and systems with correlated disorder
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
##### Abstract [en]

Condensed matter systems undergoing phase transitions rarely allow exact solutions. The presence of disorder renders the situation  even worse but collective Monte Carlo methods and parallel algorithms allow numerical descriptions. This thesis considers classical phase transitions in disordered spin systems in general and in effective models of superfluids with disorder and novel interactions in particular. Quantum phase transitions are considered via a quantum to classical mapping. Central questions are if the presence of defects changes universal properties and what qualitative implications follow for experiments. Common to the cases considered is that the disorder maps out correlated structures. All results are obtained using large-scale Monte Carlo simulations of effective models capturing the relevant degrees of freedom at the transition.

Considering a model system for superflow aided by a defect network, we find that the onset properties are significantly altered compared to the $\lambda$-transition in $^{4}$He. This has qualitative implications on expected experimental signatures in a defect supersolid scenario.

For the Bose glass to superfluid quantum phase transition in 2D we determine the quantum correlation time by an anisotropic finite size scaling approach. Without a priori assumptions on critical parameters, we find the critical exponent $z=1.8 \pm 0.05$ contradicting the long standing result $z=d$.

Using a 3D effective model for multi-band type-1.5 superconductors we find that these systems possibly feature a strong first order vortex-driven phase transition. Despite its short-range nature details of the interaction are shown to play an important role.

Phase transitions in disordered spin models exposed to correlated defect structures obtained via rapid quenches of critical loop and spin models are investigated. On long length scales the correlations are shown to decay algebraically. The decay exponents are expressed through known critical exponents of the disorder generating models. For cases where the disorder correlations imply the existence of a new long-range-disorder fixed point we determine the critical exponents of the disordered systems via finite size scaling methods of Monte Carlo data and find good agreement with theoretical expectations.

##### Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2015. , p. 12, 111
##### Series
TRITA-FYS, ISSN 0280-316X ; 2015:09
##### Keyword [en]
condensed matter physics, phase transitions, critical phenomena, spin models, quantum phase transitions, quantum fluids, superfluidity, superconductivity, disordered systems, Bose glass, dirty bosons, vortex pinning, statistical mechanics, Monte Carlo simulation, Wolff algorithm, classical worm algorithm, Wang-Landau algorithm
##### National Category
Condensed Matter Physics
Physics
##### Identifiers
ISBN: 978-91-7595-467-7 (print)OAI: oai:DiVA.org:kth-160929DiVA, id: diva2:792934
##### Public defence
2015-03-27, sal FB42, AlbaNova Universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
##### Note

QC 20150306

Available from: 2015-03-06 Created: 2015-03-04 Last updated: 2016-12-06Bibliographically approved
##### List of papers
1. Superfluid transition in a correlated defect network
Open this publication in new window or tab >>Superfluid transition in a correlated defect network
2013 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 87, no 21, p. 214520-Article in journal (Refereed) Published
##### Abstract [en]

Motivated by recent experiments on the possible supersolid behavior of He-4 solids at low temperature, we consider a model of superfluidity in a defected solid containing a system spanning network of correlated linear dislocations, or planar grain boundaries. Using arguments based on the Harris criterion, as well as numerical simulations, we find that such correlated quenched disorder shifts the familiar superfluid lambda transition to a new disordered universality class in which the correlation length exponent nu >= 1. This results in the temperature derivatives for the superfluid density d rho(s)/dT and for the heat capacity dc/dT remaining finite at the transition T-c, and thus a less singular transition, profoundly different from the usual lambda transition.

##### Keyword
Shear Modulus, Helium, Supersolidity, Systems, He-4
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:kth:diva-124723 (URN)10.1103/PhysRevB.87.214520 (DOI)000320999500003 ()2-s2.0-84879697641 (Scopus ID)
##### Funder
Swedish Research Council
##### Note

QC 20130730. Updated from "Manuscript" to "Article"

Available from: 2013-07-30 Created: 2013-07-29 Last updated: 2017-12-06Bibliographically approved
2. Quantum Critical Dynamics Simulation of Dirty Boson Systems
Open this publication in new window or tab >>Quantum Critical Dynamics Simulation of Dirty Boson Systems
2012 (English)In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 108, no 5Article in journal (Other academic) Published
##### Abstract [en]

Recently, the scaling result z = d for the dynamic critical exponent at the Bose glass to superfluid quantum phase transition has been questioned both on theoretical and numerical grounds. This motivates a careful evaluation of the critical exponents in order to determine the actual value of z. We study a model of quantum bosons at T = 0 with disorder in 2D using highly effective worm Monte Carlo simulations. Our data analysis is based on a finite-size scaling approach to determine the scaling of the quantum correlation time from simulation data for boson world lines. The resulting critical exponents are z = 1.8 +/- 0.05, nu = 1.15 +/- 0.03, and eta = -0.3 +/- 0.1, hence suggesting that z = 2 is not satisfied.

##### Keyword
Bose glass, Boson systems, Critical exponent, Dynamic critical exponents, Finite size scaling, Monte Carlo Simulation, Quantum correlations, Quantum critical, Quantum phase transitions, Simulation data
##### National Category
Condensed Matter Physics
##### Identifiers
urn:nbn:se:kth:diva-50468 (URN)10.1103/PhysRevLett.108.055701 (DOI)000299832900013 ()2-s2.0-84856529164 (Scopus ID)
##### Funder
Swedish e‐Science Research Center
##### Note

QC 20120328. Updated from submitted to published.

Available from: 2011-12-06 Created: 2011-12-06 Last updated: 2017-12-08Bibliographically approved
3. Fluctuation-induced first order phase transitions in type-1.5 superconductors in zero external field
Open this publication in new window or tab >>Fluctuation-induced first order phase transitions in type-1.5 superconductors in zero external field
2015 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 9, article id 094508Article in journal (Refereed) Published
##### Abstract [en]

In a single-component Ginzburg-Landau model that possesses thermodynamically stable vortex excitations, the zero-field superconducting phase transition is second order even when fluctuations are included. Beyond the mean-field approximation the transition is described in terms of proliferation of vortex loops. Here we determine the order of the superconducting transition in an effective 3D vortex-loop model for the recently proposed multi-band type-1.5 superconductors where the vortex interaction is non-monotonic, i.e., intermediate-range attractive and short-range repulsive. We find that the details of the vortex interaction, despite its short-range nature, can play an important role for the properties of the transition. In the type-1.5 regime with non-monotonic intervortex interaction, in contrast to the single-band case we find a first-order vortex-driven phase transition.

##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:kth:diva-160964 (URN)10.1103/PhysRevB.91.094508 (DOI)000351426000002 ()2-s2.0-84925859091 (Scopus ID)
##### Funder
Knut and Alice Wallenberg FoundationSwedish Research Council, 642-2013-7837, 325-2009-7664, 621-2012-3984
##### Note

QC 20150320

Available from: 2015-03-06 Created: 2015-03-06 Last updated: 2017-12-04Bibliographically approved
4. Phase transitions with critical loop disorder
Open this publication in new window or tab >>Phase transitions with critical loop disorder
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:kth:diva-160965 (URN)
##### Note

QS 2015

Available from: 2015-03-06 Created: 2015-03-06 Last updated: 2015-03-06Bibliographically approved
5. Phase transitions in systems with critical cluster defects
Open this publication in new window or tab >>Phase transitions in systems with critical cluster defects
##### National Category
Physical Sciences
##### Identifiers
urn:nbn:se:kth:diva-160966 (URN)
##### Note

QS 2015

Available from: 2015-03-06 Created: 2015-03-06 Last updated: 2015-03-06Bibliographically approved

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Cite
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