### Browsing by Author "Eltsov, V. B."

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Item AC Josephson effect between two superfluid time crystals(Nature Publishing Group, 2021-02) Autti, S.; Heikkinen, P. J.; Mäkinen, J. T.; Volovik, G. E.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsQuantum time crystals are systems characterized by spontaneously emerging periodic order in the time domain(1). While originally a phase of broken time translation symmetry was a mere speculation(2), a wide range of time crystals has been reported(3-5). However, the dynamics and interactions between such systems have not been investigated experimentally. Here we study two adjacent quantum time crystals realized by two magnon condensates in superfluid(3)He-B. We observe an exchange of magnons between the time crystals leading to opposite-phase oscillations in their populations-a signature of the AC Josephson effect(6)-while the defining periodic motion remains phase coherent throughout the experiment. Our results demonstrate that time crystals obey the general dynamics of quantum mechanics and offer a basis to further investigate the fundamental properties of these phases, opening pathways for possible applications in developing fields, such as quantum information processing. Two adjacent quantum time crystals implemented by two magnon condensates in the superfluid B-phase of helium-3 are observed to coherently exchange magnons as a manifestation of the AC Josephson effect, offering insights on the dynamics and interactions between these phases of matter.Item Amplitude of Waves in the Kelvin-wave Cascade(MAIK NAUKA/INTERPERIODICA/SPRINGER, 2020-04) Eltsov, V. B.; L’vov, V. S.; Department of Applied Physics; Weizmann Institute of ScienceDevelopment of experimental techniques to study superfluid dynamics, in particular, application of nanomechanical oscillators to drive vortex lines, enables potential observation of the Kelvin-wave cascade on quantized vortices. One of the first questions that then arises in analysis of the experimental results is the relation between the energy flux in the cascade and the amplitude of the Kelvin waves. We provide such relation based on the L’vov—Nazarenko picture of the cascade. Remarkably, the dependence of the amplitude of the waves on the energy flux is extremely weak as a power law with an exponent of 1/10.Item Bose-Einstein Condensation of Magnons and Spin Superfluidity in the Polar Phase of HE 3(2018-07-12) Autti, S.; Dmitriev, V. V.; Mäkinen, J. T.; Rysti, J.; Soldatov, A. A.; Volovik, G. E.; Yudin, A. N.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum Fluids; Russian Academy of SciencesThe polar phase of He3, which is topological spin-triplet superfluid with the Dirac nodal line in the spectrum of Bogoliubov quasiparticles, has been recently stabilized in a nanoconfined geometry. We pump magnetic excitations (magnons) into the sample of polar phase and observe how they form a Bose-Einstein condensate, revealed by coherent precession of the magnetization of the sample. Spin superfluidity, which supports this coherence, is associated with the spontaneous breaking of U(1) symmetry by the phase of precession. We observe the corresponding Nambu-Goldstone boson and measure its mass emerging when applied rf field violates the U(1) symmetry explicitly. We suggest that the magnon BEC in the polar phase is a powerful probe for topological objects such as vortices and solitons and topological nodes in the fermionic spectrum.Item Dimensional control of tunneling two-level systems in nanoelectromechanical resonators(American Physical Society, 2022-01-15) Kamppinen, T.; Mäkinen, J. T.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsTunneling two-level systems affect damping, noise, and decoherence in a wide range of devices, including nanoelectromechanical resonators, optomechanical systems, and qubits. Theoretically, this interaction is usually described within the tunneling state model. The dimensions of such devices are often small compared to the relevant phonon wavelengths at low temperatures, and extensions of the theoretical description to reduced dimensions have been proposed, but lack conclusive experimental verification. We have measured the intrinsic damping and the frequency shift in magnetomotively driven aluminum nanoelectromechanical resonators of various sizes at millikelvin temperatures. We find good agreement of the experimental results with a model where the tunneling two-level systems couple to flexural phonons that are restricted to one or two dimensions by geometry of the device. This model can thus be used as an aid when optimizing the geometrical parameters of devices affected by tunneling two-level systems.Item Effects of 4He Film on Quartz Tuning Forks in 3He at Ultra-low Temperatures(SPRINGER/PLENUM PUBLISHERS, 2019-07-15) Riekki, T. S.; Rysti, J.; Mäkinen, J. T.; Sebedash, A. P.; Eltsov, V. B.; Tuoriniemi, J. T.; Department of Applied Physics; Microkelvin investigations; Topological Quantum FluidsIn pure superfluid 3He–B at ultra-low temperatures, the quartz tuning fork oscillator response is expected to saturate when the dissipation caused by the superfluid medium becomes substantially smaller than the internal dissipation of the oscillator. However, even with a small amount of 4He covering the surfaces, we have observed saturation already at significantly higher temperatures than anticipated, where we have other indicators to prove that the 3He liquid is still cooling. We found that this anomalous behavior has a rather strong pressure dependence, and it practically disappears above the crystallization pressure of 4He. We also observed a maximum in the fork resonance frequency at temperatures where the transition in quasiparticle flow from the hydrodynamic to the ballistic regime is expected. We suggest that such anomalous features derive from the superfluid 4He film on the oscillator surface.Item Kelvin-Helmholtz instability of AB interface in superfluid He 3(American Physical Society, 2019-02-11) Eltsov, V. B.; Gordeev, A.; Krusius, M.; Topological Quantum Fluids; Aalto University; Department of Applied PhysicsThe Kelvin-Helmholtz instability is well known in classical hydrodynamics where it explains the sudden emergence of interfacial surface waves as a function of the flow velocity parallel to the interface. It can be carried over to the inviscid two-fluid dynamics of superfluids, to describe the stability of the phase boundary separating two bulk phases of superfluid He3 in rotating flow when the boundary is localized with a magnetic-field gradient. The results from extensive measurements as a function of temperature and pressure confirm that in the superfluid the classic condition for stability is changed and that the magnetic polarization of the B phase at the phase boundary has to be taken into account, which yields the magnetic-field-dependent interfacial surface tension.Item Lessons from topological superfluids(EDP SCIENCES, 2019-09-01) Eltsov, V. B.; Nissinen, J.; Volovik, G. E.; Department of Applied Physics; Topological Quantum FluidsAll realistic second order phase transitions are undergone at finite transition rate and are therefore non-adiabatic. In symmetry-breaking phase transitions the non-adiabatic processes, as predicted by Kibble and Zurek [1, 2], lead to the formation of topological defects (the so-called Kibble-Zurek mechanism). The exact nature of the resultingdefects depends on the detailed symmetry-breaking pattern.Item Light Higgs channel of the resonant decay of magnon condensate in superfluid He-3-B(2016-01) Zavyalov, Vladislav; Autti, S.; Eltsov, V. B.; Heikkinen, P. J.; Volovik, G. E.; Department of Applied Physics; Topological Quantum FluidsIn superfluids the order parameter, which describes spontaneous symmetry breaking, is an analogue of the Higgs field in the Standard Model of particle physics. Oscillations of the field amplitude are massive Higgs bosons, while oscillations of the orientation are massless Nambu-Goldstone bosons. The 125 GeV Higgs boson, discovered at Large Hadron Collider, is light compared with electroweak energy scale. Here, we show that such light Higgs exists in superfluid He-3-B, where one of three Nambu-Goldstone spin-wave modes acquires small mass due to the spin-orbit interaction. Other modes become optical and acoustic magnons. We observe parametric decay of Bose-Einstein condensate of optical magnons to light Higgs modes and decay of optical to acoustic magnons. Formation of a light Higgs from a Nambu-Goldstone mode observed in He-3-B opens a possibility that such scenario can be realized in other systems, where violation of some hidden symmetry is possible, including the Standard Model.Item Magnon Bose-Einstein condensates : From time crystals and quantum chromodynamics to vortex sensing and cosmology(American Institute of Physics, 2024-03-04) Mäkinen, J. T.; Autti, S.; Eltsov, V. B.; Department of Applied Physics; OtaNano; Topological Quantum FluidsUnder 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.Item Mutual friction in superfluid He- B 3 in the low-temperature regime(2018-01-29) Mäkinen, J. T.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsWe measure the response of a rotating sample of superfluid He-B3 to spin-down to rest in the zero-temperature limit. Deviations from perfect cylindrical symmetry in the flow environment cause the initial response to become turbulent. The remaining high polarization of vortices along the rotation axis suppresses the turbulent behavior and leads to laminar late-time response. We determine the dissipation during laminar decay at (0.13-0.22)Tc from the precession frequency of the remnant vortex cluster. We extract the mutual friction parameter α and confirm that its dependence on temperature and pressure agrees with theoretical predictions. We find that the zero-temperature extrapolation of α has pressure-independent value α(T=0)∼5×10-4, which we attribute to a process where Kelvin waves, excited at surfaces of the container, propagate into the bulk and enhance energy dissipation via overheating vortex core-bound fermions.Item Nanomechanical Resonators for Cryogenic Research(SPRINGER/PLENUM PUBLISHERS, 2018-12-17) Kamppinen, T.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsSuspended aluminum nanoelectromechanical resonators have been fabricated, and the manufacturing process is described in this work. Device motion is driven and detected with a magnetomotive method. The resonance response has been measured at 4.2 K temperature in vacuum and low-pressure 4He gas. At low oscillation amplitudes, the resonance response is linear, producing Lorentzian line shapes, and Q values up to 4400 have been achieved. At higher oscillation amplitudes, the devices show nonlinear Duffing-like behavior. The devices are found to be extremely sensitive to pressure in 4He gas. Such device is a promising tool for studying properties of superfluid helium.Item Nonlinear two-level dynamics of quantum time crystals(Nature Publishing Group, 2022-06-02) Autti, S.; Heikkinen, P. J.; Nissinen, J.; Mäkinen, J. T.; Volovik, G. E.; Zavyalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum Fluids; Quantum Circuits and CorrelationsA time crystal is a macroscopic quantum system in periodic motion in its ground state. In our experiments, two coupled time crystals consisting of spin-wave quasiparticles (magnons) form a macroscopic two-level system. The two levels evolve in time as determined intrinsically by a nonlinear feedback, allowing us to construct spontaneous two-level dynamics. In the course of a level crossing, magnons move from the ground level to the excited level driven by the Landau-Zener effect, combined with Rabi population oscillations. We demonstrate that magnon time crystals allow access to every aspect and detail of quantum-coherent interactions in a single run of the experiment. Our work opens an outlook for the detection of surface-bound Majorana fermions in the underlying superfluid system, and invites technological exploitation of coherent magnon phenomena – potentially even at room temperature.Item Observation of a Time Quasicrystal and Its Transition to a Superfluid Time Crystal(2018-05-25) Autti, S.; Eltsov, V. B.; Volovik, G. E.; Department of Applied Physics; Topological Quantum FluidsWe report experimental realization of a quantum time quasicrystal and its transformation to a quantum time crystal. We study Bose-Einstein condensation of magnons, associated with coherent spin precession, created in a flexible trap in superfluid He3-B. Under a periodic drive with an oscillating magnetic field, the coherent spin precession is stabilized at a frequency smaller than that of the drive, demonstrating spontaneous breaking of discrete time translation symmetry. The induced precession frequency is incommensurate with the drive, and hence, the obtained state is a time quasicrystal. When the drive is turned off, the self-sustained coherent precession lives a macroscopically long time, now representing a time crystal with broken symmetry with respect to continuous time translations. Additionally, the magnon condensate manifests spin superfluidity, justifying calling the obtained state a time supersolid or a time supercrystal.Item Observation of Half-Quantum Vortices in Topological Superfluid 3He(2016-12-14) Autti, S.; Dmitriev, V. V.; Mäkinen, J. T.; Soldatov, A. A.; Volovik, G. E.; Yudin, A. N.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum Fluids; Russian Academy of Sciences; Moscow Institute of Physics and TechnologyOne of the most sought-after objects in topological quantum-matter systems is a vortex carrying half a quantum of circulation. They were originally predicted to exist in superfluid He3−A but have never been resolved there. Here we report an observation of half-quantum vortices (HQVs) in the polar phase of superfluid He3. The vortices are created with rotation or by the Kibble-Zurek mechanism and identified based on their nuclear magnetic resonance signature. This discovery provides a pathway for studies of unpaired Majorana modes bound to the HQV cores in the polar-distorted A phase.Item Polar Phase of 3 He in Nematic Aerogel and Quartz Tuning Fork as Sensitive Detectors of Surface Boundary Conditions(SPRINGER/PLENUM PUBLISHERS, 2022-07) Dmitriev, V. V.; Eltsov, V. B.; Rysti, J.; Soldatov, A. A.; Yudin, A. N.; Russian Academy of Sciences; Department of Applied PhysicsA new superfluid phase of 3He, a polar phase, which is known to stabilize in nematic aerogel, has already led to the observation of many new phenomena. One important condition for the existence of the polar phase is a 4He coverage, which removes solid paramagnetic 3He from the surface of the aerogel strands. We report here the results of NMR experiments with superfluid 3He in nematic aerogel demonstrating the influence of 4He coverage. Simultaneous measurements of resonance properties of a quartz tuning fork immersed in liquid 3He well correlate with the NMR data.Item Propagation of self-localized Q-ball solitons in the 3He universe(2018-01-22) Autti, S.; Heikkinen, P. J.; Volovik, G. E.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsIn relativistic quantum field theories, compact objects of interacting bosons can become stable owing to conservation of an additive quantum number Q. Discovering such Q balls propagating in the universe would confirm supersymmetric extensions of the standard model and may shed light on the mysteries of dark matter, but no unambiguous experimental evidence exists. We have created long-lived Q-ball solitons in superfluid He3, where the role of the Q ball is played by a Bose-Einstein condensate of magnon quasiparticles. The principal qualitative attribute of a Q ball is observed experimentally: its propagation in space together with the self-created potential trap. Additionally, we show that this system allows for a quantitatively accurate representation of the Q-ball Hamiltonian. Our Q ball belongs to the class of the Friedberg-Lee-Sirlin Q balls with an additional neutral field ζ, which is provided by the orbital part of the Nambu-Goldstone mode. Multiple Q balls can be created in the experiment, and we have observed collisions between them. This set of features makes the magnon condensates in superfluid 3He a versatile platform for studies of Q-ball dynamics and interactions in three spatial dimensions.Item Rotating quantum wave turbulence(Nature Publishing Group, 2023-06) Mäkinen, J. T.; Autti, S.; Heikkinen, P. J.; Hosio, J. J.; Hänninen, R.; L’vov, V. S.; Walmsley, P. M.; Zavjalov, V. V.; Eltsov, V. B.; Department of Applied Physics; OtaNano; Topological Quantum Fluids; Weizmann Institute of Science; University of Manchester; Lancaster UniversityTurbulence under strong influence of rotation is described as an ensemble of interacting inertial waves across a wide range of length scales. In macroscopic quantum condensates, the quasiclassical turbulent dynamics at large scales is altered at small scales, where the quantization of vorticity is essential. The nature of this transition remains an unanswered question. Here we expand the concept of wave-driven turbulence to rotating quantum fluids where the spectrum of waves extends to microscopic scales as Kelvin waves on quantized vortices. We excite inertial waves at the largest scale by periodic modulation of the angular velocity and observe dissipation-independent transfer of energy to smaller scales and the eventual onset of the elusive Kelvin wave cascade at the lowest temperatures. We further find that energy is pumped to the system through a boundary layer distinct from the classical Ekman layer and support our observations with numerical simulations. Our experiments demonstrate a regime of turbulent motion in quantum fluids where the role of vortex reconnections can be neglected, thus stripping the transition between the classical and the quantum regimes of turbulence down to its constituent components.Item Spin, Orbital, Weyl and Other Glasses in Topological Superfluids(2018-12-27) Volovik, G. E.; Rysti, J.; Mäkinen, J. T.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsOne of the most spectacular discoveries made in superfluid 3He confined in a nanostructured material like aerogel or nafen was the observation of the destruction of the long-range orientational order by a weak random anisotropy. The quenched random anisotropy provided by the confining material strands produces several different glass states resolved in NMR experiments in the chiral superfluid 3He-A and in the time-reversal-invariant polar phase. The smooth textures of spin and orbital order parameters in these glasses can be characterized in terms of the randomly distributed topological charges, which describe skyrmions, spin vortices and hopfions. In addition, in these skyrmion glasses the momentum-space topological invariants are randomly distributed in space. The Chern mosaic, Weyl glass, torsion glass and other exotic topological states are examples of close connections between the real-space and momentum-space topologies in superfluid 3He phases in aerogel.Item Superfluid 4He as a rigorous test bench for different damping models in nanoelectromechanical resonators(American Physical Society, 2023-01-01) Kamppinen, T.; Mäkinen, J. T.; Eltsov, V. B.; Topological Quantum Fluids; Department of Applied PhysicsWe have used nanoelectromechanical resonators to probe superfluid 4He at different temperature regimes, spanning over four orders of magnitude in damping. These regimes are characterized by the mechanisms which provide the dominant contributions to damping and the shift of the resonance frequency: tunneling two-level systems at the lowest temperatures, ballistic phonons and rotons at few hundred mK, and laminar drag in the two-fluid regime below the superfluid transition temperature as well as in the normal fluid. Immersing the nanoelectromechanical resonators in fluid increases their effective mass substantially, decreasing their resonance frequency. Dissipationless superflow gives rise to a unique possibility to dramatically change the mechanical resonance frequency in situ, allowing rigorous tests on different damping models in mechanical resonators. We apply this method to characterize tunneling two-level system losses and magnetomotive damping in the devices.Item Suppressing the Kibble-Zurek Mechanism by a Symmetry-Violating Bias(American Physical Society, 2021-09-08) Rysti, J.; Mäkinen, J. T.; Autti, S.; Kamppinen, T.; Volovik, G. E.; Eltsov, V. B.; Department of Applied Physics; Topological Quantum FluidsThe formation of topological defects in continuous phase transitions is driven by the Kibble-Zurek mechanism. Here we study the formation of single- and half-quantum vortices during transition to the polar phase of He3 in the presence of a symmetry-breaking bias provided by the applied magnetic field. We find that vortex formation is suppressed exponentially when the length scale associated with the bias field becomes smaller than the Kibble-Zurek length. We thus demonstrate an experimentally feasible shortcut to adiabaticity - an important aspect for further understanding of phase transitions as well as for engineering applications such as quantum computers or simulators.