arXiv subjects
Publications and source records attributed to Eugene M. Chudnovsky.
Two-state systems may exhibit mechanical forces of purely quantum origin that have no counterpart in classical physics. We show that the such forces must exist in molecular magnets due to quantum tunneling between classically degenerate magnetic states. They can be observed in the presence of a microwave field when the magnet is placed in a static magnetic field with a gradient.
We study the Landau-Zener dynamics of a tunneling spin coupled to a torsional resonator. For strong spin-phonon coupling, when the oscillator frequency is large compared to the tunnel splitting, the system exhibits multiple Landau-Zener transitions. Entanglement of spin and mechanical angular momentum results in abrupt changes of oscillator dynamics which coincide in time with spin transitions. We show that a large number of spins on a single oscillator coupled only through the in-phase phonon field behaves as a single large spin, greatly enhancing the spin-phonon coupling. We compare purely quantum and semiclassical dynamics of the system and discuss their experimental realizations. An experiment is proposed in which the field sweep is used to read out the exact quantum state of the mechanical resonator.
Collapse of a skyrmion due to the discreteness of a crystal lattice in isotropic two-dimensional ferro- and antiferromagnets has been studied analytically and by numerical solution of equations of motion for up to 2000 x 2000 classical spins on a square lattice coupled via Heisenberg exchange interaction. Excellent agreement between analytical and numerical results has been achieved. The lifetime of the skyrmion scales with its initial size, lambda_0, as(lambda_0/a)^5 in ferromagnets and as (lambda_0/a)^2.15 in antiferromagnets, with a being the lattice parameter. This makes antiferromagnetic skyrmions significantly shorter lived than ferromagnetic skyrmions.
We study spin tunneling in a magnetic nanoparticle with biaxial anisotropy that is free to rotate about its anisotropy axis. Exact instanton of the coupled equations of motion is found that connects degenerate classical energy minima. We show that mechanical freedom of the particle renormalizes magnetic anisotropy and increases the tunnel splitting.
We study spin-rotation effects in a magnetic molecule bridged between two conducting leads. Dynamics of the total angular momentum couples spin tunneling to the mechanical rotations. Landau-Zener spin transition produced by the time-dependent magnetic field generates a unique pattern of mechanical oscillations that can be detected by measuring the electronic tunneling current through the molecule.
An extension of Drude model is proposed that accounts for spin and spin-orbit interaction of charge carriers. Spin currents appear due to combined action of the external electric field, crystal field and scattering of charge carriers. The expression for spin Hall conductivity is derived for metals and semiconductors that is independent of the scattering mechanism. In cubic metals, spin Hall conductivity $σ_s$ and charge conductivity $σ_c$ are related through $σ_s = [2 π\hbar/(3mc^2)]σ_c^2$ with $m$ being the bare electron mass. Theoretically computed value is in agreement with experiment.
Decoherence of a flux qubit due to inelastic scattering of thermal phonons by the qubit is studied. The computed decoherence rates contain no unknown constants and are expressed entirely in terms of measurable parameters of the qubit. The answer depends strongly on the size of the qubit as compared to the wavelength of phonons of frequency corresponding to the tunnel splitting of the qubit. Thermal phonons set the upper limit on the operating temperature of a small qubit at around 10-20 K. For large qubits acoustic decoherence due to one phonon processes should be taken into account at any temperature when a high quality factor is desired.
The Comment of Openov (PRL 93, 158901 (2004)) on my Physical Review Letter (PRL 92, 120405 (2004)) does not affect the universality of the decoherence mechanism described in the Letter.
We study the magnetization reversal and electromagnetic radiation due to collective Landau-Zener relaxation in a crystal of molecular magnets. Analytical and numerical solutions for the time dependence of the relaxation process are obtained. The power of the radiation and the total emitted energy are computed as functions of the crystal parameters and the field sweep rate.
Symmetry implications for the decoherence of quantum oscillations of a two-state system in a solid are studied. When the oscillation frequency is small compared to the Debye frequency, the universal lower bound on the decoherence due to the atomic environment is derived in terms of the macroscopic parameters of the solid, with no unknown interaction constants.
Spatial dependence of the magnetic field and the superconducting current in a flux line pinned by a screw dislocation are computed. Interaction of a superconducting vortex with the chiral-symmetry breaking elastic strain of a screw dislocation results in a helical current along the axis of the dislocation. It is argued that screw dislocations make impossible a force-free arrangement of flux lines in the presence of a transport current.
The prospect of developing magnetic qubits is discussed. The first part of the article makes suggestions on how to achieve the coherent quantum superposition of spin states in small ferromagnetic clusters, weakly uncompensated antiferromagnetic clusters, and magnetic molecules. The second part of the article deals with mechanisms of decoherence expected in magnetic systems. Main decohering effects are coming from nuclear spins and magnetic fields. They can be reduced by isotopic purification and superconducting shielding. In that case the time reversal symmetry of spin Hamiltonians makes spin-phonon coupling ineffective in destroying quantum coherence.
We study macroscopic quantum tunneling of the magnetic moment in a single-domain particle placed above the surface of a superconductor. Such a setup allows one to manipulate the height of the energy barrier, preserving the degeneracy of the ground state. The tunneling amplitude and the effect of the dissipation in the superconductor are computed.
The translational order in a three-dimensional lattice pinned by hopping defects is studied. It is suggested that the equilibrium state of the lattice crosses from a glass to a crystal on the rate of hopping. It is argued that such a transition may exist in flux lattices of high-temperature superconductors due to the hopping of oxygen.
Using Monte Carlo simulations, we study the properties of an elastic triangular lattice subject to a random background potential. As the cooling rate is reduced, we observe a rather sudden crossover between two different glass phases, one with exponential decay of correlations, the other with power-law decay. Contrary to predictions derived from continuum models, no evidence of a crossover in the mean-square displacement, B(r), from quadratic growth at small r, to logarithmic growth at large r is found.
We study a harmonic triangular lattice, which relaxes in the presence of a weak, short-wavelength periodic potential. Monte Carlo simulations reveal that the elastic lattice has only short-ranged positional correlations, despite the absence of defects in either lattice. Long-range orientational order, however, persists in the presence of the background. Our results provide an alternative explanation for the hexatic glassphase observed in high-temperature superconductors, magnetic bubble lattices, and charge density waves in semiconductors.