arXiv subjects

Yu. M. Bunkov

Publications and source records attributed to Yu. M. Bunkov.

At least 19 recordsLinked to original sources

arXiv2023

Direct observation of magnon BEC in an out-of-plane magnetized yttrium iron garnet film

Bose-Einstain condensation occurs at an appropriate density of bosonic particles, depending on their mass and temperature. We were able to experimentally observe the transition from the spin wave regime to the magnon Bose-Einstein condensed state (mBEC) with increasing magnon density by a microwave pumping. We used optical methods to register the spatial distribution of the magnon density and phase. For the first time, a coherent state of stationary magnons was demonstrated far from the region of their excitation.

cond-mat.mes-hall↗

arXiv2021

Micromagnetic modeling of magnon coherent states in a nonuniform magnetic field

The study of the dynamics of magnetically ordered states in strong excitation through micromagnetic modeling has become relevant due to the observation of magnon Bose condensation. In particular, the question has arisen about the possibility of describing the coherent quantum state by the quasi-classical Landau-Lifshitz-Gilbert equations. We performed micromagnetic simulations of magnetization precession with a high angle of deviation in an out-of-plane nonuniform dc field. Our results confirm the formation of coherent magnon state under conditions of high excitation. This coherent state extends over long distances and described by a spatially inhomogeneous amplitude and a homogeneous precession phase.

cond-mat.quant-gas↗

arXiv2020

Bose condensation and spin superfluidity of magnons in a perpendicularly magnetized film of yttrium iron garnet

The formation of a Bose condensate of magnons in a perpendicularly magnetized film of yttrium iron garnet under radio-frequency pumping in a strip line is studied experimentally. The characteristics of nonlinear magnetic resonance and the spatial distribution of the Bose condensate of magnons in the magnetic field gradient are investigated. In these experiments, the Bosonic system of magnons behaves similarly to the Bose condensate of magnons in the antiferromagnetic superfluid 3He-B, which was studied in detail earlier. Magnonic BEC forms a coherently precessing state with the properties of magnonic superfluidity. Its stability is determined by the repulsive potential between excited magnons, which compensates for the inhomogeneity of the magnetic field.

cond-mat.mes-hall↗

arXiv2019

The magnonic superfluid droplet at room temperature

We declare the observation of spin superfluid state in Yttrium Iron Garnet (YIG) at room temperature. It is similar to a Homogeneous Precessing State (HPD), observed earlier in antiferromagnetic superfluid $^3$He-B. The formation of this state explains by the repulsive interaction between magnons, which is required as a prior condition for the spin superfluidity. It establishes an energy gap, which stabilizes the long range superfluid transport of magnetization and determines the Ginzburg-Landau coherence length. This discovery paves a way to many quantum applications of supermagnonics at room temperature, such as magnetic Josephson effect, long distance spin transport, Q-bit, quantum logics, magnetic sensors and others.

cond-mat.soft↗

arXiv2018

Conventional magnon BEC in YIG film

The conventional magnon Bose-Einstein condensation (BEC of magnons with k = 0) is a coherent state of excited magnons described by a common wave function. It was observed first in antiferromagnetic superfluid states of 3He. Here we report on the discovery of a very similar magnon BEC in ferrimagnetic film at room temperature. The experiments were performed in Yttrium Iron Garnet (YIG) films at a magnetic field oriented perpendicular to the film. The high-density quasiequilibrium state of excited magnon was formed by methods of pulse and/or Continuous Waves (CW) magnetic resonance. We have observed a Long Lived Induction Decay Signals (LLIDS), well known as a signature of spin superfluidity. We demonstrate that the BEC state may maintain permanently by continuous replenishment of magnons with a small radiofrequency (RF) field. Our finding opens the way for development of potential supermagnonic applications at an ambient conditions.

cond-mat.quant-gas↗

arXiv2018

On the nonlinear NMR and magnon BEC in antiferromagnetic materials with coupled electron-nuclear spin precession

We present a new study of nonlinear NMR and Bose-Einstein Condensation (BEC) of nuclear spin waves in antiferromagnetic MnCO3 with coupled electron and nuclear spins. In particular, we show that the observed behaviour of NMR signals strongly contradicts the conventional description of paramagnetic ensembles of noninteracting spins based on the phenomenological Bloch equations. We present a new theoretical description of the coupled electron-nuclear spin precession, which takes into account an indirect relaxation of nuclear spins via the electron subsystem. We show that the magnitude of the nuclear magnetization is conserved for arbitrary large excitation powers, which is drastically different from the conventional heating scenario derived from the Bloch equations. This provides strong evidence that the coherent precession of macroscopic nuclear magnetization observed experimentally can be identified with BEC of nuclear spin waves with k=0.

cond-mat.other↗

arXiv2015

In-situ comprehensive calibration of a tri-port nano-electro-mechanical device

We report on experiments performed in vacuum and at cryogenic temperatures on a tri-port nano-electro-mechanical (NEMS) device. One port is a very non-linear capacitive actuation, while the two others implement the magnetomotive scheme with a linear input force port and a (quasi-linear) output velocity port. We present an experimental method enabling a full characterization of the nanomechanical device harmonic response: the non-linear capacitance function $C(x)$ is derived, and the normal parameters $k$ and $m$ (spring constant and mass) of the mode under study are measured through a careful definition of the motion (in meters) and of the applied forces (in Newtons). These results are obtained with a series of purely electric measurements performed without disconnecting/reconnecting the device, and rely only on known DC properties of the circuit, making use of a thermometric property of the oscillator itself: we use the Young modulus of the coating metal as a thermometer, and the resistivity for Joule heating. The setup requires only three connecting lines without any particular matching, enabling the preservation of a high impedance NEMS environment even at MHz frequencies. The experimental data are fit to a detailed electrical and thermal model of the NEMS device, demonstrating a complete understanding of its dynamics. These methods are quite general and can be adapted (as a whole, or in parts) to a large variety of elecromechanical devices.

cond-mat.mes-hall↗

arXiv2015

Nonlinear parametric amplification in a tri-port nanoelectromechanical device

We report on measurements performed at low temperatures on a nanoelectromechanical system (NEMS) under (capacitive) parametric pumping. The excitations and detection schemes are purely electrical, and enable in the present experiment the straightforward measurement of forces down to about a femtonewton, for displacements of an Angström, using standard room temperature electronics. We demonstrate that a small (linear) force applied on the device can be amplified up to more than a 100 times, while the system is {\it truly moving}. We explore the dynamics up to about 50$~$nm deflections for cantilevers about 200$~$nm thick by 3$~$$μ$m long oscillating at a frequency of 7$~$MHz. We present a generic modeling of nonlinear parametric amplification, and give analytic theoretical solutions enabling the fit of experimental results. We finally discuss the practical limits of the technique, with a particular application: the measurement of {\it anelastic damping} in the metallic coating of the device with an exceptional resolution of about 0.5$~$\%.

cond-mat.mes-hall↗

arXiv2012

Spin superfluidity and magnon BEC

The spin superfluidity -- superfluidity in the magnetic subsystem of a condensed matter - is manifested as the spontaneous phase-coherent precession of spins first discovered in 1984 in 3He-B. This superfluid current of spins - spin supercurrent - is one more representative of superfluid currents known or discussed in other systems, such as the superfluid current of mass and atoms in superfluid 4He; superfluid current of electric charge in superconductors; superfluid current of hypercharge in Standard Model of particle physics; superfluid baryonic current and current of chiral charge in quark matter; etc. Spin superfluidity can be described in terms of the Bose condensation of spin waves - magnons. We discuss different phases of magnon superfluidity, including those in magnetic trap; and signatures of magnons superfluidity: (i) spin supercurrent, which transports the magnetization on a macroscopic distance more than 1 cm long; (ii) spin current Josephson effect which shows interference between two condensates; (iii) spin current vortex - a topological defect which is an analog of a quantized vortex in superfluids, of an Abrikosov vortex in superconductors, and cosmic strings in relativistic theories; (iv) Goldstone modes related to the broken U(1) symmetry - phonons in the spin-superfluid magnon gas; etc. We also touch the topic of spin supercurrent in general including spin Hall and intrinsic quantum spin Hall effects.

cond-mat.other↗

arXiv2011

Self-localization of magnon Bose-Einstein condensates in the ground state and on excited levels: from harmonic to box-like trapping potential

Long-lived coherent spin precession of 3He-B at low temperatures around 0.2 Tc is a manifestation of Bose-Einstein condensation of spin-wave excitations or magnons in a magnetic trap which is formed by the order-parameter texture and can be manipulated experimentally. When the number of magnons increases, the orbital texture reorients under the influence of the spin-orbit interaction and the profile of the trap gradually changes from harmonic to a square well, with walls almost impenetrable to magnons. This is the first experimental example of Bose condensation in a box. By selective rf pumping the trap can be populated with a ground-state condensate or one at any of the excited energy levels. In the latter case the ground state is simultaneously populated by relaxation from the exited level, forming a system of two coexisting condensates.

cond-mat.quant-gas↗

arXiv2009

Magnon BEC and Spin Superfluidity: a 3He primer

Bose-Einstein condensation (BEC) is a quantum phenomenon of formation of the collective quantum state, in which the macroscopic number of particles occupies the lowest energy state and thus is governed by a single wave function. Here we highlight the BEC in a magnetic subsystem -- the BEC of magnons, elementary magnetic excitations. 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 the inhomogeneous magnetic field. We consider this phenomenon on example of spin precession in superfluid phases of $^3$He. The magnon BEC in these phases has all the properties of spin superfluidity. The states of the phase-coherent precession belong to the class of the coherent quantum states, which manifest themselves by superfluidity, superconductivity, quantum Hall effect, Josephson effect and many other macroscopic quantum phenomena.

cond-mat.quant-gas↗

arXiv2009

Magnon BEC in superfluid 3He-A

The new mode of magnetization precession in superfluid 3He-A in a squeezed aerogel has been recently reported. We consider this mode in terms of the Bose-Einstein condensation (BEC) of magnons. The difference between magnon BEC states in 3He-A and in 3He-B is discussed.

cond-mat.other↗

arXiv2008

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

Deformation of aerogel strongly modifies the orientation of the order parameter of superfluid 3He confined in aerogel. We used a radial squeezing of aerogel to keep the orbital angular momentum of the 3He 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 3He-A with a clear experimental evidence of the destruction of the long-range order by random anisotropy -- the Larkin-Imry-Ma effect. In 3He-B we observed and identified new modes of NMR, which are impossible to obtain in bulk 3He-B. One of these modes is characterized by a repulsive interaction between magnons, which is suitable for the magnon Bose-Einstein condensation (BEC).

cond-mat.soft↗

arXiv2008

Observation of Coherent Precession of Magnetization in Superfluid 3He A-phase

We report the first observation of coherent quantum precession of magnetization (CQP) in superfluid 3He-A in aerogel. The coherent precession in bulk 3He A-phase is unstable due to the positive feedback of spin supercurrent to the gradient of phase of precession. It was predicted that the homogeneous precession will be stable if the orbital momentum of 3He-A could be oriented along the magnetic field. We have succeeded to prepare this configuration by emerging 3He in uniaxially-deformed anisotropic aerogel. The dissipation rate of coherent precession states in aerogel is much larger then one in bulk 3He-B. We propose a mechanism of this dissipation.

cond-mat.other↗

arXiv2007

Electron - nuclear recoil discrimination by pulse shape analysis

In the framework of the ``ULTIMA'' project, we use ultra cold superfluid 3He bolometers for the direct detection of single particle events, aimed for a future use as a dark matter detector. One parameter of the pulse shape observed after such an event is the thermalization time constant. Until now it was believed that this parameter only depends on geometrical factors and superfluid 3He properties, and that it is independent of the nature of the incident particles. In this report we show new results which demonstrate that a difference for muon- and neutron events, as well as events simulated by heater pulses exist. The possibility to use this difference for event discrimination in a future dark matter detector will be discussed.

physics.ins-det↗

arXiv2007

Spin Vortex in Magnon BEC of Superfluid 3He-B

The phenomenon of the spontaneous 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 magnon BEC states have been observed: homogeneously precessing domain (HPD); BEC condensation in the spin-orbit potential trap (Q-balls); coherent precession with fractional magnetization; and two modes of the coherent precession in squeezed aerogel. The spin superfluidity effects, like spin Josephson phenomena, spin current vortices, spin phase slippage, long distance magnetization transport by spin supercurrents have been observed.

cond-mat.other↗

arXiv2007

Bose-Einstein condensation of magnons in superfluid 3He

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 two new modes of the coherent precession in compressed aerogel. In compressed aerogel the coherent precession has been also found in 3He-A. 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.

cond-mat.other↗

arXiv2007

Magnon condensation into Q-ball in 3He-B

The theoretical prediction of Q-balls in relativistic quantum fields is realized here experimentally in superfluid 3He-B. The condensed-matter analogs of relativistic Q-balls are responsible for an extremely long lived signal of magnetic induction -- the so-called Persistent Signal -- observed in NMR at the lowest temperatures. This Q-ball is another representative of a state with phase coherent precession of nuclear spins in 3He-B, similar to the well known Homogeneously Precessing Domain which we interpret as Bose condensation of spin waves -- magnons. At large Q the effect of self-localization is observed. In the language of relativistic quantum fields it is caused by interaction between the charged and neutral fields, where the neutral field provides the potential for the charged one. In the process of self-localization the charged field modifies locally the neutral field so that the potential well is formed in which the charge is condensed.

cond-mat.soft↗