Bohr-van Leeuwen Theorem Research Articles

Lasting effects of static magnetic field on classical Brownian motion.

The Bohr-Van Leeuwen theorem states that an external static magnetic field does not influence the state of a classical equilibrium system: There is no equilibrium classical magnetism, since the magnetic field does not do work. We revisit this famous no-go result and consider a classical charged Brownian particle interacting with an equilibrium bath. We confirm that the Bohr-Van Leeuwen theorem holds for the long-time (equilibrium) state of the particle. But the external static, homogeneous magnetic field does influence the long-time state of the thermal bath, which is described via the Caldeira-Leggett model. In particular, the magnetic field induces an average angular momentum for the (uncharged) bath, which separates into two sets rotating in opposite directions. The effect relates to the bath going slightly out of equilibrium under the influence of the Brownian particle and persists for arbitrarily long times. In this context we studied the behavior of the two other additive integrals of motion, energy, and linear momentum. The situation with linear momentum is different, because it is dissipated away by (and from) the bath modes. The average energy of the bath mode retains the magnetic field as a small correction. Thus, only the bath angular momentum really feels the magnetic field for long times.

Normal modes, rotational inertia, and thermal fluctuations of trapped ion crystals

The normal modes of a trapped ion crystal are derived using an approach based on the Hermitian properties of the system's dynamical matrix. This method is equivalent to the standard Bogoliubov method, but for classical systems, it is arguably simpler and more general in that canonical coordinates are not necessary. The theory is developed for stable, unstable, and neutrally stable systems. The method is then applied to ion crystals in a Penning trap. Reduced eigenvalue problems for the case of large applied magnetic fields are developed, for which the spectrum breaks into E × B drift modes, axial modes, and cyclotron modes. Thermal fluctuation levels in these modes are analyzed and shown to be consistent with the Bohr–van-Leeuwen theorem, provided that neutrally stable modes associated with crystal rotations are included in the analysis. An expression for the rotational inertia of the crystal is derived, and a magnetic contribution to this inertia, which dominates in large magnetic fields, is described. An unusual limit is discovered for the special case of spherically symmetric confinement, in which the rotational inertia does not exist and changes in angular momentum leave the rotation frequency unaffected.

Diamagnetic behavior in random classical radiation

Calculations for diamagnetic behavior involving Faraday induction appear in classical electromagnetism textbooks. These calculations give the charged particle motions correctly but then inaccurately introduce the statement that diamagnetism is incompatible with classical thermodynamics, and that quantum theory is required for diamagnetic behavior. Actually, if classical radiative equilibrium in classical zero-point radiation holds before the application of a magnetic field, then it will hold afterwards and will preserve the diamagnetic behavior obtained by the application of Faraday's law. Here, we consider the classical diamagnetism of a charged particle in an isotropic harmonic potential which follows from the four famous spectra of random classical radiation. The zero-point radiation spectrum fully justifies the analysis appearing in the textbooks of classical electromagnetism and in the work of Langevin. The Rayleigh-Jeans spectrum gives no diamagnetic behavior, as is consistent with the Bohr-van Leeuwen theorem. The Planck spectrum without zero-point radiation (surprisingly) gives no magnetic moment at low temperature and paramagnetic behavior at high temperature! Finally, the Planck spectrum with zero-point radiation gives diamagnetic behavior at low temperature and no magnetic moment at high temperature. This last result is in agreement with elementary quantum theory. Once again the Planck spectrum with zero-point radiation provides the best classical description.

Bohr–van Leeuwen theorem in non-commutative space

Bohr–van Leeuwen theorem has been studied in non-commutative space where the space coordinates do not commute. It has been found that in non-commutative space Bohr–van Leeuwen theorem, in general, is not satisfied and a classical treatment of the partition function of charged particles in a magnetic field gives rise to non zero magnetization.

Classical Orbital Paramagnetism in Non-equilibrium Steady State

We report the results of our numerical simulation of classical-dissipative dynamics of a charged particle subjected to a non-markovian stochastic forcing. We find that the system develops a steady-state orbital magnetic moment in the presence of a static magnetic field. Very significantly, the sign of the orbital magnetic moment turns out to be {\it paramagnetic} for our choice of parameters, varied over a wide range. This is shown specifically for the case of classical dynamics driven by a Kubo-Anderson type non-markovian noise. Natural spatial boundary condition was imposed through (1) a soft (harmonic) confining potential, and (2) a hard potential, approximating a reflecting wall. There was no noticeable qualitative difference. What appears to be crucial to the orbital magnetic effect noticed here is the non-markovian property of the driving noise chosen. Experimental realization of this effect on the laboratory scale, and its possible implications are briefly discussed. We would like to emphasize that the above steady-state classical orbital paramagnetic moment complements, rather than contradicts the Bohr-van Leeuwen (BvL) theorem on the absence of classical orbital diamagnetism in thermodynamic equilibrium.

A rigorous proof of the Bohr–van Leeuwen theorem in the semiclassical limit

The original formulation of the Bohr–van Leeuwen (BvL) theorem states that, in a uniform magnetic field and in thermal equilibrium, the magnetization of an electron gas in the classical Drude–Lorentz model vanishes identically. This stems from classical statistics which assign the canonical momenta all values ranging from -∞ to ∞ that makes the free energy density magnetic-field-independent. When considering a classical (Maxwell–Boltzmann) interacting electron gas, it is usually admitted that the BvL theorem holds upon condition that the potentials modeling the interactions are particle-velocities-independent and do not cause the system to rotate after turning on the magnetic field. From a rigorous viewpoint, when treating large macroscopic systems, one expects the BvL theorem to hold provided the thermodynamic limit of the free energy density exists (and the equivalence of ensemble holds). This requires suitable assumptions on the many-body interactions potential and on the possible external potentials to prevent the system from collapsing or flying apart. Starting from quantum statistical mechanics, the purpose of this paper is to give, within the linear-response theory, a proof of the BvL theorem in the semiclassical limit when considering a dilute electron gas in the canonical conditions subjected to a class of translational invariant external potentials.

Thermally driven ratchet motion of a skyrmion microcrystal and topological magnon Hall effect

Spontaneously emergent chirality is an issue of fundamental importance across the natural sciences. It has been argued that a unidirectional (chiral) rotation of a mechanical ratchet is forbidden in thermal equilibrium, but becomes possible in systems out of equilibrium. Here we report our finding that a topologically nontrivial spin texture known as a skyrmion--a particle-like object in which spins point in all directions to wrap a sphere--constitutes such a ratchet. By means of Lorentz transmission electron microscopy we show that micrometre-sized crystals of skyrmions in thin films of Cu2OSeO3 and MnSi exhibit a unidirectional rotation motion. Our numerical simulations based on a stochastic Landau-Lifshitz-Gilbert equation suggest that this rotation is driven solely by thermal fluctuations in the presence of a temperature gradient, whereas in thermal equilibrium it is forbidden by the Bohr-van Leeuwen theorem. We show that the rotational flow of magnons driven by the effective magnetic field of skyrmions gives rise to the skyrmion rotation, therefore suggesting that magnons can be used to control the motion of these spin textures.

Meissner effect, diamagnetism, and classical physics—a review

We review the literature on what classical physics says about the Meissner effect and the London equations. We discuss the relevance of the Bohr-van Leeuwen theorem for the perfect diamagnetism of superconductors and conclude that the theorem is based on invalid assumptions. We also point out results in the literature that show how magnetic flux expulsion from a sample cooled to superconductivity can be understood as an approach to the magnetostatic energy minimum. These results have been published several times but many textbooks on magnetism still claim that there is no classical diamagnetism, and virtually all books on superconductivity repeat Meissner’s 1933 statement that flux expulsion has no classical explanation.

Classical orbital magnetic moment in a dissipative stochastic system

We present an analytical treatment of the dissipative-stochastic dynamics of a charged classical particle confined biharmonically in a plane with a uniform static magnetic field directed perpendicular to the plane. The stochastic dynamics gives a steady state in the long-time limit. We have examined the orbital magnetic effect of introducing a parametrized deviation (η-1) from the second fluctuation-dissipation relation that connects the driving noise and the frictional memory kernel in the standard Langevin dynamics. The main result obtained here is that the moving charged particle generates a finite orbital magnetic moment in the steady state, and that the moment shows a crossover from para- to diamagnetic sign as the parameter η is varied. It is zero for η=1 that makes the steady state correspond to equilibrium, as it should. The magnitude of the orbital magnetic moment turns out to be a nonmonotonic function of the applied magnetic field, tending to zero in the limit of an infinitely large as well as an infinitesimally small magnetic field. These results are discussed in the context of the classic Bohr-van Leeuwen theorem on the absence of classical orbital diamagnetism. Possible realization is also briefly discussed.

CLASSICAL DIAMAGNETISM REVISITED

The well-known Bohr–van Leeuwen Theorem states that the orbital diamagnetism of classical charged particles is identically zero in equilibrium. However, results based on real space–time approach using the classical Langevin equation predicts non-zero diamagnetism for classical unbounded (finite or infinite) systems. Here we show that the recently discovered Fluctuation Theorems, namely, the Jarzynski Equality or the Crooks Fluctuation Theorem surprisingly predicts a free energy that depends on magnetic field as well as on the friction coefficient, in outright contradiction to the canonical equilibrium results. However, in the cases where the Langevin approach is consistent with the equilibrium results, the Fluctuation Theorems lead to results in conformity with equilibrium statistical mechanics. The latter is demonstrated analytically through a simple example that has been discussed recently.

Bohr–van Leeuwen theorem and the thermal Casimir effect for conductors

The problem of estimating the thermal corrections to Casimir and Casimir-Polder interactions in systems involving conducting plates has attracted considerable attention in the recent literature on dispersion forces. Alternative theoretical models, based on distinct low-frequency extrapolations of the plates reflection coefficient for transverse electric (TE) modes, provide widely different predictions for the magnitude of this correction. In this paper we examine the most widely used prescriptions for this reflection coefficient from the point of view of their consistency with the Bohr-van Leeuwen theorem of classical statistical physics, stating that at thermal equilibrium transverse electromagnetic fields decouple from matter in the classical limit. We find that the theorem is satisfied if and only if the TE reflection coefficient vanishes at zero frequency in the classical limit. This criterion appears to rule out some of the models that have been considered recently for describing the thermal correction to the Casimir pressure with non-magnetic metallic plates.

Charged particle in a magnetic field: Jarzynski equality

We describe some solvable models which illustrate the Jarzynski theorem and related fluctuation theorems. We consider a charged particle in the presence of a magnetic field in a two-dimensional harmonic well. In the first case the center of the harmonic potential is translated with a uniform velocity, while in the other case the particle is subjected to an ac force. We show that the Jarzynski identity complements the Bohr-van Leeuwen theorem on the absence of diamagnetism in an equilibrium classical system.

Classical statistical thermodynamics of a gas of charged particles in magnetic field

We will demonstrate that the paradox of classical statistical thermodynamics for a gas of charged particles in a magnetic field (GMF) has not yet been explained. The paradox lies in the statement that the average magnetic moment of a gas is zero, whereas the time-average magnetic moment of each particle is always negative. We consider a gas of charged particles moving in a plane perpendicular to a uniform magnetic field. The density of distribution of the ensemble describing statistical properties of the GMF is derived starting from the basics, with due regard for the specific character of dynamics of the charged particles in the magnetic field. It is emphasized that neither the imposition of a potential barrier restricting the existence region of the GMF, nor the introduction of a background neutralizing charge occupying a finite area, is a necessary condition for the stationary equilibrium state of the GMF to exist. We show that the reason for this fact is that the density of distribution of the ensemble is dependent, besides the Hamiltonian, on another positive definite integral of motion that is a linear combination of the Hamiltonian and the angular momentum of the GMF. Basic thermodynamic relations are deduced in terms of the new density of distribution, and it is demonstrated that the GMF has a magnetic moment whose magnitude and sign are determined by the external potential field. Particularly, the GMF is diamagnetic in the absence of the neutralizing background charge. Thus, the statement of the Bohr-van Leeuwen theorem, deduced using the ordinary Gibbs density of distribution depending on the Hamiltonian only, is wrong. It is noted that a great deal of works on the theory of electronic phenomena in magnetic field are based either on the same wrong density of distribution or on the formula for average occupation numbers depending on the energy of states, which follows from this density of distribution within quantum theory. These theories should be revised in view of the new form of the density of distribution.

On the decoupling between classical Coulomb matter and radiation

We consider a model of matter coupled to radiation at equilibrium. Matter is described by a one-component plasma of classical point charges with Coulomb interactions, while radiation is represented by the classical transverse potential vector in Coulomb gauge. Using a straightforward generalization of the Bohr–van Leeuwen theorem, we show that the equilibrium properties of classical Coulomb matter remain unaffected by the presence of the classical radiation. As far as the real world is concerned, this decoupling does survive at large distances where both matter and radiation can be treated classically. This invalidates all the large-distances predictions, for the charge correlations, of the so-called Darwin models which incorporate retarded electromagnetic interactions beyond the instantaneous Coulomb potential. A second related important consequence is that the first relativistic corrections to the Coulomb thermodynamical quantities must be evaluated within the theory of quantum electrodynamics at finite temperature, even in a weakly relativistic and almost classical regime for matter.