Yuriy M. Bunkov

Strong orientational effect of stretched aerogel on the 3 He order parameter

Deformation of aerogel strongly modifies the orientation of the order parameter of superfluid (3)He confined in aerogel. We used a radial squeezing of aerogel to keep the orbital angular momentum of the (3)He Cooper pairs in the plane perpendicular to the magnetic field. We did not find strong evidence for a polar phase, with a nodal line along the equator of the Fermi surface, predicted to occur at large radial squeezing. Instead we observed (3)He-A with a clear experimental evidence of the destruction of the long-range order by random anisotropy-the Larkin-Imry-Ma effect. In (3)He-B we observed and identified new modes of NMR, which are impossible to obtain in bulk (3)He-B. One of these modes is characterized by a repulsive interaction between magnons, which is suitable for the magnon Bose-Einstein condensation.

Bose-Einstein Condensation of Magnons in Superfluid 3He

Abstract The possibility of Bose-Einstein condensation of excitations has been discussed for a long time. The phenomenon of the phase-coherent precession of magnetization in superfluid 3He and the related effects of spin superfluidity are based on the true Bose-Einstein condensation of magnons. Several different states of coherent precession has been observed in 3He-B: homogeneously precessing domain (HPD); persistent signal formed by Q-balls at very low temperatures; coherent precession with fractional magnetization; and a mode of the coherent precession in compressed aerogel. The coherent precession has been also found in 3He-A in compressed aerogel. Here we demonstrate that all these cases are examples of a Bose-Einstein condensation of magnons, with the magnon interaction term in the Gross-Pitaevskii equation being provided by different types of spin-orbit coupling in the background of the coherent precession.

Magnon Bose–Einstein condensation and spin superfluidity

Bose-Einstein condensation (BEC) is a quantum phenomenon of formation of a collective quantum state in which a macroscopic number of particles occupy the lowest energy state and thus is governed by a single wavefunction. Here we highlight the BEC in a magnetic subsystem--the BEC of magnons, elementary magnetic excitations. The magnon BEC is manifested as the spontaneously emerging state of the precessing spins, in which all spins precess with the same frequency and phase even in an inhomogeneous magnetic field. The coherent spin precession was observed first in superfluid (3)He-B and this domain was called the homogeneously precessing domain (HPD). The main feature of the HPD is the induction decay signal, which ranges over many orders of magnitude longer than is prescribed by the inhomogeneity of magnetic field. This means that spins precess not with a local Larmor frequency, but coherently with a common frequency and phase. This BEC can also be created and stabilized by continuous NMR pumping. In this case the NMR frequency plays the role of a magnon chemical potential, which determines the density of the magnon condensate. The interference between two condensates has also been demonstrated. It was shown that HPD exhibits all the properties of spin superfluidity. The main property is the existence of a spin supercurrent. This spin supercurrent flows separately from the mass current. Transfer of magnetization by the spin supercurrent by a distance of more than 1 cm has been observed. Also related phenomena have been observed: the spin current Josephson effect; the phase-slip processes at the critical current; and the spin current vortex--a topological defect which is the analog of a quantized vortex in superfluids and of an Abrikosov vortex in superconductors; and so on. It is important to mention that the spin supercurrent is a magnetic phenomenon, which is not directly related to the mass superfluidity of (3)He: it is the consequence of a specific antiferromagnetic ordering in superfluid (3)He. Several different states of coherent precession have been observed in (3)He-B: the homogeneously precessing domain (HPD); a persistent signal formed by Q-balls at very low temperatures; coherent precession with fractional magnetization; and two new modes of coherent precession in compressed aerogel. In compressed aerogel the coherent precession has been also found in (3)He-A. We demonstrate that the coherent precession of magnetization is a true BEC of magnons, with the magnon interaction term in the Gross-Pitaevskii equation being provided by spin-orbit coupling which is different for different states of the magnon BEC.

Spin superfluidity and coherent spin precession

The spontaneous phase coherent precession of the magnetization in superfluid (3)He-B was discovered experimentally in 1984 at the Institute for Physical Problems, Moscow by Borovik-Romanov, Bunkov, Dmitriev and Mukharsky and simultaneously explained theoretically by Fomin (Institut Landau, Moscow). Its formation is a direct manifestation of spin superfluidity. The latter is the magnetic counterpart of mass superfluidity and superconductivity. It is also an example of the Bose-Einstein condensation of spin-wave excitations (magnons). The coherent spin precession opened the way for investigations of spin supercurrent magnetization transport and other related phenomena, such as spin-current Josephson effect, process of phase slippage at a critical value of spin supercurrent, spin-current vortices, non-topological solitons (analogous to Q-balls in high energy physics) etc. New measuring techniques based on coherent spin precession made the investigation of mass counterflow and mass vortices possible owing to the spin-mass interaction. New phenomena were observed: mass-spin vortices, the Goldstone mode of the mass vortex with non-axisymmetric core, superfluid density anisotropy etc. Different types of coherent spin precession were later found in superfluid (3)He-A and (3)He-B confined in anisotropic aerogel, in the states with counterflow and in (3)He with reduced magnetization. Finally, spin superfluidity investigations developed the basis for a modern investigation of electron spin supercurrent and spintronics.

Metallic coatings of microelectromechanical structures at low temperatures: Stress, elasticity, and nonlinear dissipation

We present mechanical measurements performed at low temperatures on cantilever-based microelectromechanical structures coated with a metallic layer. Two very different coatings are presented in order to illustrate the capabilities of the present approach, namely (soft) aluminum and (hard) niobium oxide. The temperature is used as a control parameter to access materials properties. We benefit from low temperature techniques to extract a phase-resolved measurement of the first mechanical resonance mode in cryogenic vacuum. By repeating the experiment on the same samples, after multiple metallic depositions, we can determine accurately the contribution of the coating layers to the mechanical properties in terms of surface stress, additional mass, additional elasticity, and damping. Analytic theoretical expressions are derived and used to fit the data. Taking advantage of the extremely broad dynamic range provided by the technique, we can measure the anelasticity of the thin metallic film. The key parameters ...

Topological defects and coherent magnetization precession of 3He in aerogel

Abstract We have observed for the first time the formation of the region with coherent precession of magnetization (CPM) in superfluid 3 He in aerogel covered by solid monolayers of 4 He . The signal of CPM has been observed by pulsed NMR and by CW NMR. By playing with the magnetic field gradient we were able to distinguish the spatial position of the region of CPM as well as to determine the long-scale inhomogenity of the order parameter. We have observed an evidence for existence of an enormous number of topological defects in superfluid 3 He in aerogel.

Surface instability of coherent precession in the non-hydrodynamic regime

We have studied numerically the dynamics of the order parameter of3He−B undergoing coherent quantum precession near the walls of a cell. We have found that the Brinkman-Smith configuration is perturbed by the interaction of the orbital component of the order parameter with the walls. This perturbation is responsible for the ultralow temperature instability of the coherent precession in the nonhydrodynamic regime.

Bolometric calibration of a superfluid 3He detector for Dark Matter search: Direct measurement of the scintillated energy fraction for neutron, electron and muon events

Abstract We report on the calibration of a superfluid 3 He bolometer developed for the search of non-baryonic Dark Matter. Precise thermometry is achieved by the direct measurement of thermal excitations using Vibrating Wire Resonators (VWRs). The heating pulses for calibration were produced by the direct quantum process of quasiparticle generation by other VWRs present. The bolometric calibration factor is analyzed as a function of temperature and excitation level of the sensing VWR. The calibration is compared to bolometric measurements of the nuclear neutron capture reaction and heat depositions by cosmic muons and low energy electrons. The comparison allows a quantitative estimation of the ultra-violet scintillation rate of irradiated helium, demonstrating the possibility of efficient electron recoil event rejection.

Observation of a new relaxation mechanism in3He-B

Abstract Homogeneously precessing domain (HPD) has been studied by CW NMR at low pressures and temperatures down to 0.28 T c . We observed, inconsistently to existing theory, a large absorption term linear with HPD length and inversely proportional to the temperature.

Superfluid 3He—from cosmology to particle detection

Abstract We describe the application of superfluid 3 He at very low temperatures as a highly sensitive bolometer for elementary particles detection. Recently we were able to detect the cosmic muon scattering with energy of 45 KeV . The other important property of superfluid 3 He at very low temperatures is its analogy with quantum vacuum of the Universe. At very rapid superfluid transition in 3 He , follows after a reaction with single neutron, the creation of topological defects (vortices) has been demonstrated in accordance with the Kibble–Zurek scenario for the cosmological analogue. We discuss here the extension of the Kibble–Zurek scenario for the case when alternative symmetries may be broken and different states can be nucleated independently. We have calculated the nucleation probability of the various states of superfluid 3 He during a superfluid transition. The new theory of transition from supercooled A phase to the B phase, triggered by nuclear reaction, have been established. Our theory explains the results of Stanford experiments much better, then well known “Baked Alaska” scenario.