Relaxation and decoherence of orbital and spin degrees of freedom in quantum dots

The phonon induced mechanisms of relaxation/decoherence in quantum dots are analysed. A non-perturbative technique - a modification of the Davydov transformation appropriate to the localised particles is applied for solving the electron-phonon eigenvalue problem in a quantum dot at magnetic field presence. The decay rates for polaron relaxation via the anharmonicity induced channel are analysed in details. In particular, it is indicated that previous, of perturbative type, estimations of the anharminicity induced relaxation rates were too severe and after including the coherence effects they are of, at least, one order longer. The process of exciton dressing with phonons is also analysed as the unavoidable source of picosecond scale decoherence in optically driven nanostructures. A break-down of an instant Pauli spin blocking mechanism and a large enhancement of the Fröhlich constant for confined electrons are also addressed.     

Lattice dynamics of LaMnO3: Coupling of the lattice and orbital degrees of freedom

The lattice dynamics of regular LaMnO₃ is calculated within a shell model with pairwise interionic interaction potentials and with a Jahn-Teller (JT) contribution included into the energy and dynamic matrix of the crystal. A correlation is made between Raman spectral lines and lattice vibrations. The positions of some lines in the Raman spectrum are found to depend heavily on the linear JT coupling constant V _e . The effect of the JT coupling on the phonon density of states of LaMnO₃ is investigated.     

Coupling between internal spin dynamics and external degrees of freedom in the presence of colored noise

We observe asymmetric transition rates between Zeeman levels (spin flips) of magnetically trapped atoms. The asymmetry strongly depends on the spectral shape of an applied noise. This effect follows from the interplay between the internal states of the atoms and their external degrees of freedom, where different trapped levels experience different potentials. Such insight may prove useful for controlling atomic states by the introduction of noise, as well as provide a better understanding of the effect of noise on the coherent operation of quantum systems.     

Interrelation of the spin,orbital, and charge degrees of freedom in half-doped manganites

Based on the double-exchange model in the tight-binding approximation, self-consistent energy-band-structure calculations are carried out for the first time for the orbital and charge ordering in La_0.5Ca_0.5MnO₃-type manganites with 16 Mn ions in the unit cell under the assumption of a linear dependence of the Jahn-Teller splitting of the e _g level on the occupancy of each ion. The equilibrium magnetic and orbital configurations are determined by minimizing the total energy with respect to the direction of the local magnetic moments and the orbital states of the Mn ions. The dependence of the splitting on the occupancy favors the stabilization of phases with charge separation. This separation is an important factor determining the magnetic structure of the ground state. The ground state can be an insulating antiferromagnetic structure of the CE or G type or a new ferrimagnetic structure with a magnetization lower than that of a saturated ferromagnet by a factor of 2. The existence and the width of the bandgap in the electronic energy spectrum of the CE phase depend on the ratio between the values of the Hund exchange and the splitting. When the splitting is sufficiently large, the Jahn-Teller effect stabilizes the insulating state. The finite value of the Hund coupling also favors the stabilization of the CE phase for the realistic values of the interionic exchange and the hopping integral of itinerant electrons.     

Invariant surfaces and orbital behaviour in dynamical systems of 3 degrees of freedom. II

Two formal integrals of motion besides the Hamiltonian are found in a system of three degrees of freedom with three unperturbed frequencies ω₁, ω₂, ω₃ which are close to, or exactly at, the double resonance 6:4:3. The integrals are truncated at various orders and they are used to find theoretical invariant surfaces on a surface of section.The theoretical invariant surfaces are then compared with the empirical results. The agreement is improved as the order of the integrals increases from the 5th up to the 11th degree. This is surprising because theoretical considerations indicate that one formal integral breaks down beyond order 6, because of the second resonance 4:3. (However it is possible to construct one further integral by a different method). As expected the agreement is worse when the perturbation becomes large. A check of the constancy of the truncated integrals along various orbits shows that the relative variations become smaller as the order increases; they become larger as the perturbation increases.     

Separation of charge and spin degrees of freedom in the Hubbard model

The representation of the electron field operators separating charge from spin and providing transmutations between spin and isospin quantum numbers has been derived for any dimension. In this representation the interaction term in the Hubbard model is expressed in a bilinear form of quasiparticle field operators. The quasiparticles are spinless fermions with the electron electric charge. The consequence of this representation is that superconductivity in the strong coupling Hubbard model is due to odd wave pairings of electrons.The author expresses his sincere gratitude to Professor S. Stenholm for discussions and for the warm hospitality at the Research Institute for Theoretical Physics, University of Helsinki where this work has began. He would especially like to thank Professor C. Cronström who played an important role in the early and final stages of this work. Professor Anderson is acknowledged for providing the author with two chapters of his book (not yet published) quoted in the Discussion.     

Dependence of effective mass on spin and valley degrees of freedom

We measure the effective mass (m) of interacting two-dimensional electrons confined to an AlAs quantum well while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively. Compared to its band value, m is enhanced unless the electrons are fully valley- and spin-polarized. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior.     

On the partial trace over collective spin degrees of freedom

We derive analytical properties for the degeneracy ν(N,j) occurring in the decomposition of the state space C2⊗N. We also investigate the dynamics of two qubits coupled via Ising interactions to separate spin baths, and we study the thermodynamic limit.     

Narrowing of topological bands due to electronic orbital degrees of freedom

The fractional quantum Hall effect has been predicted to occur in the absence of magnetic fields and at high temperature in lattice systems that have flat bands with a nonzero Chern number. We demonstrate that orbital degrees of freedom in frustrated lattice systems lead to a narrowing of topologically nontrivial bands. This robust effect does not rely on fine-tuned long-range hopping parameters and is directly relevant to a wide class of transition-metal compounds.     

Heat transfer between two degrees of freedom

A particularly simple chaotic nonequilibrium open system with two Cartesian degrees of freedom, characterized by two distinct temperatures T(x) and T(y), is introduced. The two temperatures are maintained by Nose-Hoover canonical-ensemble thermostats. Both the equilibrium (no net heat transfer) and nonequilibrium (dissipative) Lyapunov spectra are characterized for this simple system.     

Local degrees of freedom and nuclear quadrupolar spin relaxation in liquid metals and alloys

We discuss the temperature dependence of the nuclear quadrupolar spin relaxation rate in metals and alloys. Four different relaxation processes must be distinguished: Hard collisions, diffusion, decay of associates and rotational diffusion. The latter two processes are important in alloys of strongly interacting alloying partners, the former in pure metals (collision) and ideal or regular mixtures. Diffusion is important if the alloying partners produce different electrical field gradients.     

反铁磁体KCuF3中的孤立子能量

考虑磁-声子耦合作用和磁子间相互作用,采用双子格模型和相干态表示,研究了自旋S=1/2反铁磁体KCuF3的非线性集体激发特性,求出了孤立子的能量和质量,并比较了磁性孤立子和局域性磁子激发能量。结果表明在一维反铁磁体KCuF3中发现孤立子激发是可能。     

Interplay between orbital and spin motion in heavy nuclei

We investigate the role of orbital and spin motion in shaping the structure of the magnetic dipole and quadrupole spectra in spherical as well as in deformed heavy nuclei.     

Spin, charge, orbital and lattice degrees of freedom in quasi-cubic manganites: A brief review

The rich phase diagram of La_1-xCa_xMnO₃ is the consequence of the interplay of spin, charge, orbital and lattice degrees of freedom, giving rise to several magnetic phases, Jahn–Teller distortions, spin-canting and phase separation, and orbital and charge order. Each Mn ion is in a mixed valent state of two magnetic configurations, Mn⁴⁺ and Mn³⁺. We study the phase diagram using mean-field slave bosons for the itinerant e_g-electrons in two orbitals with excluded multiple occupancy of each site, Hund's rule interaction between the e_g and t_2g states and Jahn–Teller coupling of the e_g orbitals to the lattice.     

Collectivity and single-particle degrees of freedom

Intruder rotational bands in ⁴⁵Sc and ⁴⁵Ti have been investigated up to the maximum aligned angular momentum by means of EUROBALL IV and the Recoil Filter Detector (RFD). The use of the RFD allowed for a reduction of the -line Doppler broadening and, moreover, for a determination of very short level lifetimes. In the studied nuclei, the estimated deformation shows a gradual disappearance of the collectivity at the highest available spins.Received: 19 November 2002, Revised: 14 March 2003, Published online: 2 March 2004PACS: 21.10.Tg Lifetimes - 21.60.Cs Shell model - 27.40. + z P. Bednarczyk: Present address: GSI, Darmstadt, GermanyM.B. Smith: Present address: TRIUMF, Vancouver, Canada     

Orbital order fluctuations in KCuF3

The interaction of orbital momenta of copper in KCuF₃ via the field of elastic deformations is analyzed taking into account the local rotations of electron density on copper ions. It is found that the interaction energy related to local rotations of copper ions is comparable in order of magnitude with the Dzyaloshinskii-Moriya interaction energy. The calculated interaction energy of copper orbitals in the ab plane of the crystal exceeds the energy of their interaction along the c axis. This circumstance explains recently found specific features of orbital melting in LaMnO₃ with increasing temperature.     

Dynamical Dzyaloshinsky-Moriya interaction in KCuF3

The spin dynamics of the prototypical quasi-one-dimensional antiferromagnetic Heisenberg spin S=1/2 chain KCuF3 is investigated by electron spin resonance spectroscopy. Our analysis shows that the peculiarities of the spin dynamics require a new dynamical form of the antisymmetric anisotropic spin-spin interaction. This dynamical Dzyaloshinsky-Moriya interaction is related to strong oscillations of the bridging fluorine ions perpendicular to the crystallographic c axis. This new mechanism allows us to resolve consistently the controversies in observation of the magnetic and structural properties of this orbitally ordered perovskite compound.     

Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO3/SrTiO3 superlattices

We report first-principles calculations on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO(3))(n)/(SrTiO(3))(n) superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions can be used to control the orientation of the main axis of the system. The electrostatic constrains at the interface facilitate the polarization rotation and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those required considering only strain effects. In addition, ferroelectric (FE) and antiferrodistortive (AFD) modes are strongly coupled. Usual steric arguments cannot explain this coupling and a covalent model is proposed to account for it. The energy gain due to the FE-AFD coupling decreases with the periodicity of the superlattice, becoming negligible for n ≥ 3.