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Quantum Critical Dynamics Simulation of Dirty Boson Systems
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.ORCID iD: 0000-0003-1164-0831
2012 (English)In: Physical Review Letters, ISSN 0031-9007, 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.

##### Place, publisher, year, edition, pages
2012. Vol. 108, no 5
##### Keyword [en]
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
ISI: 000299832900013ScopusID: 2-s2.0-84856529164OAI: oai:DiVA.org:kth-50468DiVA: diva2:461977
##### 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: 2015-03-06Bibliographically approved
##### In thesis
1. Superfluid Phase Transitions in Disordered Systems
Open this publication in new window or tab >>Superfluid Phase Transitions in Disordered Systems
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:nbn:se:kth:diva-50051 (URN)978-91-7501-199-9 (ISBN)
##### Presentation
2011-12-15, FA 32, AlbaNova, Roslagstullsbacken 21, Stockholm, 10:18 (English)
##### Note

QC 20111206

Available from: 2011-12-06 Created: 2011-12-01 Last updated: 2015-03-05Bibliographically approved
2. Phase transitions in novel superfluids and systems with correlated disorder
Open this publication in new window or tab >>Phase transitions in novel superfluids and systems with correlated disorder
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. 12, 111 p.
##### Series
TRITA-FYS, ISSN 0280-316X ; 2015:09
##### Keyword
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
urn:nbn:se:kth:diva-160929 (URN)978-91-7595-467-7 (ISBN)
##### 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: 2015-03-06Bibliographically approved

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