Abstract
Symmetry and its breaking are central to the modern understanding of the physical world. In conjunction with a handful of judiciously chosen experiments and topological reasoning, they have guided us to formulating fundamental laws in the context of relativistic quantum field theory as well as the theory of possible states of matter, their phase transitions and universal behavior. Topological defects, such as quantized vortices, affect the behavior at macroscopic scales. The range of possible defects is governed by the broken symmetries and topology of the system. The symmetries contained by the normal phase of ³He are exceptionally rich, and support a variety of symmetrybreaking phase transitions to macroscopically coherent, superfluid, states. Due to the existence of these symmetrybreaking phase transitions and the zoo of topological and nontopological defects allowed by the different superfluid phases, ³He can be used as a model system for a wide range of research topics ranging from turbulence and topological quantum computing to cosmology and grand unification scenarios.
This thesis focuses on experimental studies on properties of linear topological defects born due to breaking of the U(1) symmetry at the superfluid transition  quantized vortices. At ultralow temperatures in the superfluid B phase we quantify the interpretation of vortex imaging techniques based on Andreev scattering of thermal quasiparticles and study the response of an equilibrium vortex array to perturbations, namely to a rapid spindown of the rotating drive. Superfluid B phase of ³He provides a unique platform to study nonequilibrium hydrodynamics as the ratio of the inertial and viscous forces may be tuned by orders of magnitude by changing the temperature.
Recent advances in experimental techniques have demonstrated that under engineered confinement the topology of the superfluid may be altered, resulting in novel superfluid phases. These phases provide experimental access to new types of topological and nontopological defects, which are also studied in this thesis. We demonstrate that a new type of vortex  the halfquantum vortex (HQV), is stabilized in the superfluid polar phase. Taking advantage of the phase diagram of ³He under confinement, we demonstrate that HQVs may be transferred to superfluid phases with polar distortion  with striking consequences. In the polardistorted A phase the HQVs have been predicted to harbor isolated Majorana fermions that in 2D systems obey nonAbelian statistics, while in the polardistorted B phase the HQVs survive as "walls bounded by strings"  composite defects predicted in cosmological context decades ago.
Translated title of the contribution  Dynamiikka ja vorteksirakenteet topologisessa ³Hesupranesteessä ultramatalissa lämpötiloissa sekä rajoitetussa geometriassa 

Original language  English 
Qualification  Doctor's degree 
Awarding Institution 

Supervisors/Advisors 

Publisher  
Print ISBNs  9789526084114 
Electronic ISBNs  9789526084121 
Publication status  Published  2019 
MoE publication type  G5 Doctoral dissertation (article) 
Keywords
 helium3
 superfluid
 quantized vortex
 ultralow temperatures
 topological matter
 halfquantum vortex
 walls bounded by strings
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Equipment


OtaNano – Low Temperature Laboratory
Alexander Savin (Manager)
Department of Applied PhysicsFacility/equipment: Facility