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Papers by Nikos Papanicolaou

Physical Review B, 2021

We find numerically skyrmionic textures with skyrmion number Q = 0 in ferromagnets with the Dzyal... more We find numerically skyrmionic textures with skyrmion number Q = 0 in ferromagnets with the Dzyaloshinskii-Moriya interaction and perpendicular anisotropy. These have the form of a skyrmionantiskyrmion pair and may be called chiral droplets. They are stable in an infinite film as well as in disc-shaped magnetic elements. Droplets are found for values of the parameters close to the transition from the ferromagnetic to the spiral phase. We study their motion under spin-transfer torque. They move in the direction of the spin flow and, thus, their dynamics are drastically different than the Hall dynamics of the standard Q = 1 skyrmion.

New Journal of Physics, 2008

We show that transmutation of linear momentum into position may occur in a system of three magnet... more We show that transmutation of linear momentum into position may occur in a system of three magnetic vortices thanks to a direct link between topology and dynamics in a ferromagnet. This happens via exchange between the linear momentum of a vortex-antivortex pair and the position of a single vortex during a semi-elastic scattering process. Vortex polarity switching occurs in the case of inelastic collisions.

Nuclear Physics B, 1992

ABSTRACT

Annals of Physics, 1979

We develop a path integral formalism that allows for semi-classical quantization of systems with ... more We develop a path integral formalism that allows for semi-classical quantization of systems with spin degrees of freedom. We apply it to study the continuous Heisenberg spin chain, which has been known to possess interesting classical solutions. The calculated semi-classical spectrum turns out to be essentially exact. We also construct a new infinite series of conservation laws that are nonlocal generalizations of the spin.

SciPost Physics

Skyrmions in antiferromagnetic (AFM) materials with the Dzyaloshinskii-Moriya (DM) interaction ar... more Skyrmions in antiferromagnetic (AFM) materials with the Dzyaloshinskii-Moriya (DM) interaction are expected to exist for essentially the same reasons as in DM ferromagnets (FM). It is shown that skyrmions in antiferromagnets with the DM interaction can be traveling as solitary waves with velocities up to a maximum value that depends on the DM parameter. Their configuration is found numerically. The energy and the linear momentum of an AFM skyrmion lead to a proper definition of its mass. We give the details of the energy-momentum dispersion of traveling skyrmions and explore their particle-like character based on exact relations. The skyrmion number, known to be linked to the dynamics of topological solitons in FM, is, here, unrelated to the dynamical behavior. As a result, the solitonic behavior of skyrmions in AFM is in stark contrast to the dynamical behavior of their FM counterparts.

Physics Letters A

ABSTRACT

Physical Review B

The dynamics of domain walls in a model weak ferromagnet is shown to be governed by a suitable ex... more The dynamics of domain walls in a model weak ferromagnet is shown to be governed by a suitable extension of the relativistic nonlinear model to account for the Dzyaloshinskii-Moriya anisotropy and an applied magnetic field. Our analytical results are confirmed by a numerical calculation in a discrete spin model and significantly amend earlier treatments. Thus we provide a detailed description of static domain walls and subsequently study their dynamics. A virial theorem is derived that underlies the existence of a terminal state and allows a simple calculation of the mobility at low fields for both Bloch and Néel walls. We further establish the existence of a critical field above which a driven domain wall is always Néel, whereas a bifurcation takes place below the critical value where the two types of walls behave rather differently. The terminal states as well as the mobility curves are obtained for practically any strength of the applied field. Implications for the phenomenology of domain walls in orthoferrites and in rhombohedron weak ferromagnets are discussed briefly. ͓S0163-1829͑97͒10017-0͔

Physics Letters B, 1978

ABSTRACT

The dynamics of N point vortices in a fluid is described by the Helmholtz-Kirchhoff (HK) equation... more The dynamics of N point vortices in a fluid is described by the Helmholtz-Kirchhoff (HK) equations which lead to a completely integrable Hamiltonian system for N=2 or 3 but chaotic dynamics for N>3. Here we consider a generalization of the HK equations to describe the dynamics of magnetic vortices within a collective-coordinate approximation. In particular, we analyze in detail the dynamics of a system of three magnetic vortices by a suitable generalization of the solution for three point vortices in an ordinary fluid obtained by Groebli more than a century ago. The significance of our results for the dynamics of ferromagnetic elements is briefly discussed.

Physical Review B, 2015

We study the dynamics of skyrmions under spin-transfer torque in Dzyaloshinskii-Moriya materials ... more We study the dynamics of skyrmions under spin-transfer torque in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. In particular, we study the motion of a topological skyrmion with skyrmion number Q = 1 and a non-topological skyrmionium with Q = 0 using their linear momentum, virial relations, and numerical simulations. The non-topological Q = 0 skyrmionium is accelerated in the direction of the current flow and it either reaches a steady state with constant velocity, or it is elongated to infinity. The steady-state velocity is given by a balance between current and dissipation and has an upper limit. In contrast, the topological Q = 1 skyrmion converges to a steady-state with constant velocity at an angle to the current flow. When the spin current stops the Q = 1 skyrmion is spontaneously pinned whereas the Q = 0 skyrmionium continues propagation. Exact solutions for the propagating skyrmionium are identified as solutions of equations given numerically in a previous work. Further exact results for propagating skyrmions are given in the case of the pure exchange model. The traveling solutions provide arguments that a spin polarized current will cause rigid motion of a skyrmion or a skyrmionium.

Physical Review B, 1997

ABSTRACT We present a theoretical study of electron-spin resonance (ESR) in spin-1 chains with a ... more ABSTRACT We present a theoretical study of electron-spin resonance (ESR) in spin-1 chains with a strong planar anisotropy and an exchange interaction that is either ferromagnetic or antiferromagnetic. For a sufficiently weak bias field, the ground state carries vanishing azimuthal spin and the resonance spectrum is dominated by transitions to excitons and antiexcitons with azimuthal spin ±1. When the field exceeds a certain critical value, the ground state is a fully saturated ferromagnet and the relevant elementary excitations are ordinary magnons. The contribution of a two-magnon bound state, often referred to as the single-ion bound state, is of special importance and is analyzed in detail. These theoretical results are consistent with available ESR measurements on Ni(C2H8N2)2Ni(CN)4 and suggest more detailed experiments.

Physical Review B, 2015

We study the dynamics of skyrmions in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. ... more We study the dynamics of skyrmions in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. An important link between topology and dynamics is established through the construction of unambiguous conservation laws obtained earlier in connection with magnetic bubbles and vortices. In particular, we study the motion of a topological skyrmion with skyrmion number Q = 1 and a non-topological skyrmionium with Q = 0 under the influence of an applied field gradient. The Q = 1 skyrmion undergoes Hall motion perpendicular to the direction of the field gradient with a drift velocity proportional to the gradient. In contrast, the non-topological Q = 0 skyrmionium is accelerated in the direction of the field gradient, thus exhibiting ordinary Newtonian motion. When the applied field is switched off the Q = 1 skyrmion is spontaneously pinned around a fixed guiding center, whereas the Q = 0 skyrmionium moves with constant velocity v. We give a systematic calculation of a skyrmionium traveling with any constant velocity v that is smaller than a critical velocity vc.

Singularities in Fluids, Plasmas and Optics, 1993

Physical review. B, Condensed matter, 1995

ABSTRACT

Springer Tracts in Modern Physics, 1994

A direct link between the topological complexity of magnetic media and their dynamics is establis... more A direct link between the topological complexity of magnetic media and their dynamics is established through the construction of unambiguous conservation laws for the linear and angular momenta as moments of a topological vorticity. As a consequence, the dynamics of topological magnetic solitons is shown to exhibit the characteristic features of the Hall effect of electrodynamics or the Magnus effect of fluid dynamics. The main points of this program are reviewed here for both ferromagnets and antiferromagnets, while a straightforward extension to the study of superfluids is also discussed briefly.

Physical Review B, 2002

The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spi... more The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value Hc. The T = 0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear σ model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H1 and H2 such that H1 < Hc < H2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.

2011 IEEE SENSORS Proceedings, 2011

The fabrication of a power-free, reversible, colorindicator sensor of the relative humidity in th... more The fabrication of a power-free, reversible, colorindicator sensor of the relative humidity in the environment has numerous applications and is considered a challenging task. The device to be developed should be able of color changing according to the degree of humidity change and have a number of characteristics, i.e., reversibility, fast response, low cost, and, ideally, should be flexible. The proposed solution, based on a 1-D polymeric Photonic Crystal (PC) configuration on a microscope glass, can successfully fulfill all the above requirements. I.

Thin Solid Films, 2008

Photoelectron spectroscopy using synchrotron radiation and ab-initio electronic structure calcula... more Photoelectron spectroscopy using synchrotron radiation and ab-initio electronic structure calculations were used in order to describe the fine structure of the valence band in the Sn/Ni(111) system. The characteristic contributions of each metal in the valence band photoemission spectra obtained with a photon energy of 80 eV and their changes upon the formation of the (√3 × √3)R30°Sn/Ni(111) surface alloy were also born out in the calculated density-of-states curves in fair agreement with the experiments. The Sn-Ni interaction leads to a considerable broadening of the valence band width at the bimetallic surfaces.

Solid State Communications, 1988

Quantum fluctuations in a dimenzed electron-phonon chain are studied within the Su-Schrieffer-Hee... more Quantum fluctuations in a dimenzed electron-phonon chain are studied within the Su-Schrieffer-Heeger model using a systematic 1/n expansion. The calculated phonon spectrum differs appreciably from recent results of Rice et al., and quantum corrections to the phonon order parameter are found to be smaller than those predicted by Monte Carlo simulations.

Physical Review B, 2021

We find numerically skyrmionic textures with skyrmion number Q = 0 in ferromagnets with the Dzyal... more We find numerically skyrmionic textures with skyrmion number Q = 0 in ferromagnets with the Dzyaloshinskii-Moriya interaction and perpendicular anisotropy. These have the form of a skyrmionantiskyrmion pair and may be called chiral droplets. They are stable in an infinite film as well as in disc-shaped magnetic elements. Droplets are found for values of the parameters close to the transition from the ferromagnetic to the spiral phase. We study their motion under spin-transfer torque. They move in the direction of the spin flow and, thus, their dynamics are drastically different than the Hall dynamics of the standard Q = 1 skyrmion.

New Journal of Physics, 2008

We show that transmutation of linear momentum into position may occur in a system of three magnet... more We show that transmutation of linear momentum into position may occur in a system of three magnetic vortices thanks to a direct link between topology and dynamics in a ferromagnet. This happens via exchange between the linear momentum of a vortex-antivortex pair and the position of a single vortex during a semi-elastic scattering process. Vortex polarity switching occurs in the case of inelastic collisions.

Nuclear Physics B, 1992

ABSTRACT

Annals of Physics, 1979

We develop a path integral formalism that allows for semi-classical quantization of systems with ... more We develop a path integral formalism that allows for semi-classical quantization of systems with spin degrees of freedom. We apply it to study the continuous Heisenberg spin chain, which has been known to possess interesting classical solutions. The calculated semi-classical spectrum turns out to be essentially exact. We also construct a new infinite series of conservation laws that are nonlocal generalizations of the spin.

SciPost Physics

Skyrmions in antiferromagnetic (AFM) materials with the Dzyaloshinskii-Moriya (DM) interaction ar... more Skyrmions in antiferromagnetic (AFM) materials with the Dzyaloshinskii-Moriya (DM) interaction are expected to exist for essentially the same reasons as in DM ferromagnets (FM). It is shown that skyrmions in antiferromagnets with the DM interaction can be traveling as solitary waves with velocities up to a maximum value that depends on the DM parameter. Their configuration is found numerically. The energy and the linear momentum of an AFM skyrmion lead to a proper definition of its mass. We give the details of the energy-momentum dispersion of traveling skyrmions and explore their particle-like character based on exact relations. The skyrmion number, known to be linked to the dynamics of topological solitons in FM, is, here, unrelated to the dynamical behavior. As a result, the solitonic behavior of skyrmions in AFM is in stark contrast to the dynamical behavior of their FM counterparts.

Physics Letters A

ABSTRACT

Physical Review B

The dynamics of domain walls in a model weak ferromagnet is shown to be governed by a suitable ex... more The dynamics of domain walls in a model weak ferromagnet is shown to be governed by a suitable extension of the relativistic nonlinear model to account for the Dzyaloshinskii-Moriya anisotropy and an applied magnetic field. Our analytical results are confirmed by a numerical calculation in a discrete spin model and significantly amend earlier treatments. Thus we provide a detailed description of static domain walls and subsequently study their dynamics. A virial theorem is derived that underlies the existence of a terminal state and allows a simple calculation of the mobility at low fields for both Bloch and Néel walls. We further establish the existence of a critical field above which a driven domain wall is always Néel, whereas a bifurcation takes place below the critical value where the two types of walls behave rather differently. The terminal states as well as the mobility curves are obtained for practically any strength of the applied field. Implications for the phenomenology of domain walls in orthoferrites and in rhombohedron weak ferromagnets are discussed briefly. ͓S0163-1829͑97͒10017-0͔

Physics Letters B, 1978

ABSTRACT

The dynamics of N point vortices in a fluid is described by the Helmholtz-Kirchhoff (HK) equation... more The dynamics of N point vortices in a fluid is described by the Helmholtz-Kirchhoff (HK) equations which lead to a completely integrable Hamiltonian system for N=2 or 3 but chaotic dynamics for N>3. Here we consider a generalization of the HK equations to describe the dynamics of magnetic vortices within a collective-coordinate approximation. In particular, we analyze in detail the dynamics of a system of three magnetic vortices by a suitable generalization of the solution for three point vortices in an ordinary fluid obtained by Groebli more than a century ago. The significance of our results for the dynamics of ferromagnetic elements is briefly discussed.

Physical Review B, 2015

We study the dynamics of skyrmions under spin-transfer torque in Dzyaloshinskii-Moriya materials ... more We study the dynamics of skyrmions under spin-transfer torque in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. In particular, we study the motion of a topological skyrmion with skyrmion number Q = 1 and a non-topological skyrmionium with Q = 0 using their linear momentum, virial relations, and numerical simulations. The non-topological Q = 0 skyrmionium is accelerated in the direction of the current flow and it either reaches a steady state with constant velocity, or it is elongated to infinity. The steady-state velocity is given by a balance between current and dissipation and has an upper limit. In contrast, the topological Q = 1 skyrmion converges to a steady-state with constant velocity at an angle to the current flow. When the spin current stops the Q = 1 skyrmion is spontaneously pinned whereas the Q = 0 skyrmionium continues propagation. Exact solutions for the propagating skyrmionium are identified as solutions of equations given numerically in a previous work. Further exact results for propagating skyrmions are given in the case of the pure exchange model. The traveling solutions provide arguments that a spin polarized current will cause rigid motion of a skyrmion or a skyrmionium.

Physical Review B, 1997

ABSTRACT We present a theoretical study of electron-spin resonance (ESR) in spin-1 chains with a ... more ABSTRACT We present a theoretical study of electron-spin resonance (ESR) in spin-1 chains with a strong planar anisotropy and an exchange interaction that is either ferromagnetic or antiferromagnetic. For a sufficiently weak bias field, the ground state carries vanishing azimuthal spin and the resonance spectrum is dominated by transitions to excitons and antiexcitons with azimuthal spin ±1. When the field exceeds a certain critical value, the ground state is a fully saturated ferromagnet and the relevant elementary excitations are ordinary magnons. The contribution of a two-magnon bound state, often referred to as the single-ion bound state, is of special importance and is analyzed in detail. These theoretical results are consistent with available ESR measurements on Ni(C2H8N2)2Ni(CN)4 and suggest more detailed experiments.

Physical Review B, 2015

We study the dynamics of skyrmions in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. ... more We study the dynamics of skyrmions in Dzyaloshinskii-Moriya materials with easy-axis anisotropy. An important link between topology and dynamics is established through the construction of unambiguous conservation laws obtained earlier in connection with magnetic bubbles and vortices. In particular, we study the motion of a topological skyrmion with skyrmion number Q = 1 and a non-topological skyrmionium with Q = 0 under the influence of an applied field gradient. The Q = 1 skyrmion undergoes Hall motion perpendicular to the direction of the field gradient with a drift velocity proportional to the gradient. In contrast, the non-topological Q = 0 skyrmionium is accelerated in the direction of the field gradient, thus exhibiting ordinary Newtonian motion. When the applied field is switched off the Q = 1 skyrmion is spontaneously pinned around a fixed guiding center, whereas the Q = 0 skyrmionium moves with constant velocity v. We give a systematic calculation of a skyrmionium traveling with any constant velocity v that is smaller than a critical velocity vc.

Singularities in Fluids, Plasmas and Optics, 1993

Physical review. B, Condensed matter, 1995

ABSTRACT

Springer Tracts in Modern Physics, 1994

A direct link between the topological complexity of magnetic media and their dynamics is establis... more A direct link between the topological complexity of magnetic media and their dynamics is established through the construction of unambiguous conservation laws for the linear and angular momenta as moments of a topological vorticity. As a consequence, the dynamics of topological magnetic solitons is shown to exhibit the characteristic features of the Hall effect of electrodynamics or the Magnus effect of fluid dynamics. The main points of this program are reviewed here for both ferromagnets and antiferromagnets, while a straightforward extension to the study of superfluids is also discussed briefly.

Physical Review B, 2002

The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spi... more The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value Hc. The T = 0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear σ model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H1 and H2 such that H1 < Hc < H2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.

2011 IEEE SENSORS Proceedings, 2011

The fabrication of a power-free, reversible, colorindicator sensor of the relative humidity in th... more The fabrication of a power-free, reversible, colorindicator sensor of the relative humidity in the environment has numerous applications and is considered a challenging task. The device to be developed should be able of color changing according to the degree of humidity change and have a number of characteristics, i.e., reversibility, fast response, low cost, and, ideally, should be flexible. The proposed solution, based on a 1-D polymeric Photonic Crystal (PC) configuration on a microscope glass, can successfully fulfill all the above requirements. I.

Thin Solid Films, 2008

Photoelectron spectroscopy using synchrotron radiation and ab-initio electronic structure calcula... more Photoelectron spectroscopy using synchrotron radiation and ab-initio electronic structure calculations were used in order to describe the fine structure of the valence band in the Sn/Ni(111) system. The characteristic contributions of each metal in the valence band photoemission spectra obtained with a photon energy of 80 eV and their changes upon the formation of the (√3 × √3)R30°Sn/Ni(111) surface alloy were also born out in the calculated density-of-states curves in fair agreement with the experiments. The Sn-Ni interaction leads to a considerable broadening of the valence band width at the bimetallic surfaces.

Solid State Communications, 1988

Quantum fluctuations in a dimenzed electron-phonon chain are studied within the Su-Schrieffer-Hee... more Quantum fluctuations in a dimenzed electron-phonon chain are studied within the Su-Schrieffer-Heeger model using a systematic 1/n expansion. The calculated phonon spectrum differs appreciably from recent results of Rice et al., and quantum corrections to the phonon order parameter are found to be smaller than those predicted by Monte Carlo simulations.