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Superfluid Phase Transitions in Disordered Systems
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents results from large scale Monte Carlo simulations of systems subject to a superfluid phase transition in the presence of disorder. The simulations are performed by state-of-the-art, collective Monte Carlo algorithms treating phase degrees of freedom in effective models with amplitude fluctuations integrated out.

In Paper I a model system for the possible solid to supersolid transition in 4He is presented.The Wolff cluster algorithm is used to study how the presence of linearly correlated random defects is able to alter the universality class of the 3-dimensional XY-model. In the pure case the superfluid density and heat capacity have singular onsets, which are not seen in the supersolid experiments where instead a smooth onset is obtained. Using finite size scaling of Monte Carlo data, we find a similar smooth onset in our simulations, governed by exponents  ν=1 for the superfluid density and α=-1 for the heat capacity. These results are in qualitative agreement with experiments for the observed transition in solid 4He.

In Paper II a systematic investigation of the scaling result z=d for the dynamic critical exponentat the Bose glass to superfluid quantum phase transition is performed. The result z=d has been believed to be exact for about 20 years, but although it has been questioned lately no accurate estimate of z has been available. An effective link current model of quantum bosons at T=0 with disorder in 2D is simulated using highly effective worm Monte Carlo simulations.The data analysis is based on a finite size scaling approach todetermine the quantum correlation time from simulationdata for boson world lines without any a priori assumption on the critical parameters. The resulting critical exponents are z=1.8 \pm 0.05, ν=1.15 \pm 0.03, and η=-0.3 \pm 0.1. This suggests that z=d is not satisfied.

      

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. , 73 p.
Series
TRITA-FYS, ISSN 0280-316X ; 2011:58
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-50051ISBN: 978-91-7501-199-9 OAI: oai:DiVA.org:kth-50051DiVA: diva2:461740
Presentation
2011-12-15, FA 32, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:18 (English)
Opponent
Supervisors
Note

QC 20111206

Available from: 2011-12-06 Created: 2011-12-01 Last updated: 2015-03-05Bibliographically 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, 214520- p.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

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