A magnon Bose-Einstein condensate (BEC) in superfluid 3He is a fine instrument for studying the surrounding macroscopic quantum system. At zero temperature, the BEC is subject to a few distinct forms of decay into other collective excitations, owing to momentum and energy conservation in a quantum vacuum. We study the vortex-Higgs mechanism: The vortices relax the requirement for momentum conservation, allowing the optical magnons of the BEC to transform into light Higgs quasiparticles. This facilitates a direct measurement of the dimensions of the B-phase double-core vortex, providing experimental access to elusive phenomena, such as the Kelvin wave cascade and core-bound Majorana fermions. Our paper expands the spectrum of possible interactions between magnetic quasiparticles in 3He−B and lays the groundwork for building magnon-based quantum devices.
A magnon Bose-Einstein condensate (BEC) in superfluid 3He is a fine instrument for studying the surrounding macroscopic quantum system. At zero temperature, the BEC is subject to a few distinct forms of decay into other collective excitations, owing to momentum and energy conservation in a quantum vacuum. We study the vortex-Higgs mechanism: The vortices relax the requirement for momentum conservation, allowing the optical magnons of the BEC to transform into light Higgs quasiparticles. This facilitates a direct measurement of the dimensions of the B-phase double-core vortex, providing experimental access to elusive phenomena, such as the Kelvin wave cascade and core-bound Majorana fermions. Our paper expands the spectrum of possible interactions between magnetic quasiparticles in 3He−B and lays the groundwork for building magnon-based quantum devices.
Under suitable experimental conditions, collective spin-wave excitations, magnons, form a Bose–Einstein condensate (BEC), where the spins precess with a globally coherent phase. Bose–Einstein condensation of magnons has been reported in a few systems, including superfluid phases of 3He, solid state systems, such as yttrium-iron-garnet films, and cold atomic gases. The superfluid phases of 3He provide a nearly ideal test bench for coherent magnon physics owing to experimentally proven spin superfluidity, the long lifetime of the magnon condensate, and the versatility of the accessible phenomena. We first briefly recap the properties of the different magnon BEC systems, with focus on superfluid 3He. The main body of this review summarizes recent advances in the application of magnon BEC as a laboratory to study basic physical phenomena connecting to diverse areas from particle physics and cosmology to vortex dynamics and new phases of condensed matter. This line of research complements the ongoing efforts to utilize magnon BECs as probes and components for potentially room-temperature quantum devices. In conclusion, we provide a roadmap for future directions in the field of applications of magnon BEC to fundamental research.
In this thesis we study Bose-Einstein condensates of trapped magnon quasiparticles in superfluid 3He-B. These condensates form in the trap provided by the order-parameter texture and applied magnetic field profile. We provide numerical description of two properties of the condensates, both observed in experiments: One of these properties is the ability to interact with the trapping potential, which draws an analogy to Q-balls of high energy physics. The other is an extra relaxation mechanism owing to interaction with vortices of broken symmetry. We construct a three-dimensional model of magnon Q-balls. The spatial variation of the order parameter in equilibrium is solved in a quasi-three-dimensional model by minimizing the total free energy. Eigenstates of the resulting potential found by solution of the Schr odinger equation in three dimensions. We discuss the formation and properties of Q-balls of magnons, which are "true" Q-balls in the sense that the trapping potential is forced to follow the movement of the magnon condensate. In high energy physics Q-balls are non-topological solitons that are stable owing to the charge conservation. In the case of magnon BECs the approximately conserved magnon number represents the Q-ball charge. We calculate the NMR signal using a realistic model of the measurement circuit, and show that in the case of the magnon Q-ball the signal is not generally proportional to the transverse magnetization when the condensate moves in space. The calibration of experimental signal amplitude is further used in the description of rocking oscillations of the broken-symmetry vortex cores and related dissipation of the magnon condensate. These oscillations are driven by the precessing magnetization of the magnon condensate.
For several decades, superfluid helium has provided an almost unparalleled frontier of quantum physics research, especially in topics related to macroscopic quantum phenomena. While nowadays systems studied in quantum physics are becoming increasingly artificially engineered - the quest for quantum computers and quantum technology in general being one of the driving forces of this development - traditionally both superfluid 4He and 3He have provided a natural but versatile window to their quantum nature. Superfluid 3He, the subject of this thesis, is a quantitatively understood macroscopic quantum system of considerable yet manageable mathematical complexity. In this thesis I show how such versatility translates into a variety of emergent phenomena, touching many seemingly distant fields in physics such as cosmology.
Approaching superfluid helium-3 experimentally requires sophisticated cooling and probing methodology. We have used a rotating ultra-low-temperature refrigerator, where the superfluid sample is cooled down by a nuclear demagnetisation stage and probed using nuclear magnetic resonance spectroscopy (NMR). One particularly useful NMR instrument can be constructed by trapping a Bose-Einstein condensate (BEC) of magnon quasiparticles within the superfluid. We have used such condensates in probing a variety of delicate phenomena such as other spin wave modes, including Higgs modes. We have also observed propagation of self-trapped Q-ball solitons making use of magnon BECs. These achievements required careful experimental and numerical studies of the magnon BEC. The spectrum of the magnon BEC is controlled by the profile of the external magnetic field and by the spatial variation of the superfluid order parameter. The main part of the relaxation is due to temperature dependent spin transport in the normal component of the liquid, which allows translating the magnon BEC read-out into in-situ thermometer.
As perhaps the most pubilicized topic of this thesis, I explain how we discovered half-quantum vortices in superfluid helium-3, a manifestation of the topological versatility of this system. Nobel laureate Anthony Leggett recently listed finding these vortices the most important remaining open problem in the ultra-low-temperature superfluid physics. In general, the very name of quantum physics refers to the observation that fundamental concepts such as energy and momentum are quantised in the microscopic world, that is, changes in the quantities describing a physical system can only occur in finite steps, integer multiples one quantum. Therefore finding vortices carrying only half-a-quantum of circulation is not only intriguing, but manifests deep understanding of the underlying physics and quantum physics in general.; Supranesteet ovat jo vuosikymmenien ajan olleet kvanttifysiikan tutkimuksen kapeassa karjessa, erityisesti mita tulee makroskooppisten kvantti-ilmioiden tutkimiseen. Siina missa tamanhetkiset suuret virtaukset fysiikan tutkimuksessa kulkevat kohti varta vasten suunniteltuja kohdesysteemeja -…
Helium stays in liquid form down to absolute zero temperature. Since the effects of quantum mechanics become particularly evident at low temperatures, helium is an ideal system for experimental studies of macroscopic quantum phenomena. One of the most remarkable of these is the transition into zero-viscosity superfluid phase. In case of fermionic helium-3 isotope, this transition occurs at temperatures below 0.003 K. In the low-temperature B phase of superfluid helium-3, the transport of both mass and magnetization occur without losses. The spin superfluidity gives rise to spontaneous coherent long-living precession of magnetization observed in nuclear magnetic resonance (NMR) experiments. The coherent precession can be described in terms of spin-wave excitations, or in the language of Bose-Einstein condensation (BEC) of magnon quasiparticles. In this dissertation, we trap these quasiparticle condensates at selected locations in the superfluid order parameter texture, using both textural and magnetic energies for confinement. The sample is cooled to ultra-low temperatures in rotating adiabatic demagnetization refrigerator. Careful characterization of the creation and the decay of the trapped magnon BEC reveals nonhydrodynamic spin diffusion as the main temperature-dependent source of relaxation. Using this property, we suggest magnon-BEC-based microkelvin thermometry in helium-3 systems. Both the relaxation rate and the precession frequency of magnetization are sensitive to changes in the properties of the surrounding superfluid at the lowest temperatures and clearly more informative than conventional NMR techniques which freeze to a temperature-independent state. Here we have used the magnon BECs to detect quantized vortices, gravity waves on the free surface of superfluid, and long-living textural defects related to the creation and the annihilation of phase boundaries. We have also found enhanced zero-temperature relaxation of magnon BEC due to free surface. We propose that it originates from the bound quasiparticle states at the surface which supposedly have Majorana nature following from the topology of helium-3. Owing to diverse properties, superfluid helium-3 serves as an experimentally accessible model system for phenomena elsewhere in physics. We have found self-trapping of magnons towards a box-like confinement potential via coupling with the order parameter which resembles the Q-ball in quantum field theories. We have also observed the interplay between three low-energy spin-wave modes, one of which acquires mass due to spin-orbit interaction and becomes similar to the light Higgs particle considered in extensions of Standard Model.
For several decades, superfluid helium has provided an almost unparalleled frontier of quantum physics research, especially in topics related to macroscopic quantum phenomena. While nowadays systems studied in quantum physics are becoming increasingly artificially engineered - the quest for quantum computers and quantum technology in general being one of the driving forces of this development - traditionally both superfluid 4He and 3He have provided a natural but versatile window to their quantum nature. Superfluid 3He, the subject of this thesis, is a quantitatively understood macroscopic quantum system of considerable yet manageable mathematical complexity. In this thesis I show how such versatility translates into a variety of emergent phenomena, touching many seemingly distant fields in physics such as cosmology. Approaching superfluid helium-3 experimentally requires sophisticated cooling and probing methodology. We have used a rotating ultra-low-temperature refrigerator, where the superfluid sample is cooled down by a nuclear demagnetisation stage and probed using nuclear magnetic resonance spectroscopy (NMR). One particularly useful NMR instrument can be constructed by trapping a Bose-Einstein condensate (BEC) of magnon quasiparticles within the superfluid. We have used such condensates in probing a variety of delicate phenomena such as other spin wave modes, including Higgs modes. We have also observed propagation of self-trapped Q-ball solitons making use of magnon BECs. These achievements required careful experimental and numerical studies of the magnon BEC. The spectrum of the magnon BEC is controlled by the profile of the external magnetic field and by the spatial variation of the superfluid order parameter. The main part of the relaxation is due to temperature dependent spin transport in the normal component of the liquid, which allows translating the magnon BEC read-out into in-situ thermometer. As perhaps the most pubilicized topic of this thesis, I explain how we discovered half-quantum vortices in superfluid helium-3, a manifestation of the topological versatility of this system. Nobel laureate Anthony Leggett recently listed finding these vortices the most important remaining open problem in the ultra-low-temperature superfluid physics. In general, the very name of quantum physics refers to the observation that fundamental concepts such as energy and momentum are quantised in the microscopic world, that is, changes in the quantities describing a physical system can only occur in finite steps, integer multiples one quantum. Therefore finding vortices carrying only half-a-quantum of circulation is not only intriguing, but manifests deep understanding of the underlying physics and quantum physics in general.
The Bose–Einstein condensation (BEC) corresponds to the formation of a collective quantum state in which macroscopic number of particles is governed by a single wave function. The magnon BEC forms by excited non-equilibrium magnons and manifests itself by coherent precession of magnetization even in an inhomogeneous magnetic field. The magnon BEC is very similar to an atomic BEC, but the potential of the interaction between magnons may variate very significantly. The superfluid phases of 3He are the best antiferromagnetic system for investigations of magnon BEC and spin superfluidity. The 6 different states of magon BEC were observed in 3He. Recently magnon BEC was observed in antiferromagnets with Suhl–Nakamura interaction and ferrites. Here we review for the first time the switch off NMR method, when magnon BEC forms during a long radiofrequency pulse. The new experimental results are discussed.