Retrieval of Dispersion Dependences of Waveguide Modes from Ship Noise Measurements Using Two Synchronized Arrays

Acoustical Physics - An approach is proposed for estimating the dispersion characteristics of waveguide modes from analysis of ship noise recorded by two closely spaced and synchronized vertical...     

Giant enhancement of quantum decoherence by frustrated environments

Decoherence in a system of two antiferromagnetically coupled spins that interact with a spin bath environment is studied. Systems are considered that range from the rotationally invariant to highly anisotropic spin models and have different topologies and values of parameters that are fixed or are allowed to fluctuate randomly. We explore the conditions under which the two-spin system clearly shows an evolution from the initial spin-up-spin-down state towards the maximally entangled singlet state. We demonstrate that frustration and, especially, glassiness of the spin environment strongly enhances the decoherence of the two-spin system. The text was submitted by the authors in English.     

The effective spin hamiltonian and phase separation instability of the almost half-filled hubbard model and the narrow-band s-ƒ model

The effective spin Hamiltonian is constructed in the framework of the almost half-filled Hubbard model on the Cayley tree by means of functional integral technique with the use of static approximation. The system in the ground state appears to be consisting of the ferromagnetic metallic domains and the antiferromagnetic insulating one sprovided that the concentration of excess electrons (or holes) does not exceed some critical value. The connection between the Hubbard model and the s-? model is stated.     

Magnetic susceptibility of the spin polaron states in the s-f exchange model above curie temperature

The static spin susceptibility tensor of magnetic semiconductor in a framework of the s-f exchange model above the Curie temperature is calculated. The Feynman path integral variational method is used to take into account the spin polaron formation. Free energy estimations of the phenomenological theory of Krivoglaz are justified microscopically. The spin polaron effect on ESR frequencies is considered.     

Anharmonic magnetic deformation of self-assembled molecular nanocapsules

High magnetic fields were used to deform spherical nanocapsules, self-assembled from bolaamphiphilic sexithiophene molecules. At low fields the deformation--measured through linear birefringence-scales quadratically with the capsule radius and with the magnetic field strength. These data confirm a long standing theoretical prediction [W. Helfrich, Phys. Lett. A 43, 409 (1973)10.1016/0375-9601(73)90396-4], and permit the determination of the bending rigidity of the capsules as (2.6+/-0.8) x 10(-21) J. At high fields, an enhanced rigidity is found which cannot be explained within the Helfrich model. We propose a complete form of the free energy functional that accounts for this behavior, and allows discussion of the formation and stability of nanocapsules in solution.     

Anomalous thermal expansion in alpha-titanium

We provide a complete quantitative explanation for the anisotropic thermal expansion of hcp Ti at low temperature. The observed negative thermal expansion along the c axis is reproduced theoretically by means of a parameter free theory which involves both the electron and phonon contributions to the free energy. The thermal expansion of titanium is calculated and found to be negative along the c axis for temperatures below approximately 170 K, in good agreement with observations. We have identified a saddle point van Hove singularity near the Fermi level as the main reason for the anisotropic thermal expansion in alpha-titanium.     

Dual boson approach to collective excitations in correlated fermionic systems

We develop a general theory of a boson decomposition for both local and non-local interactions in lattice fermion models which allows us to describe fermionic degrees of freedom and collective charge and spin excitations on equal footing. An efficient perturbation theory in the interaction of the fermionic and the bosonic degrees of freedom is constructed in the so-called dual variables in the path-integral formalism. This theory takes into account all local correlations of fermions and collective bosonic modes and interpolates between itinerant and localized regimes of electrons in solids. The zero-order approximation of this theory corresponds to an extended dynamical mean-field theory (EDMFT), a regular way to calculate nonlocal corrections to EDMFT is provided. It is shown that dual ladder summation gives a conserving approximation beyond EDMFT. The method is especially suitable for consideration of collective magnetic and charge excitations and allows to calculate their renormalization with respect to “bare” RPA-like characteristics. General expression for the plasmonic dispersion in correlated media is obtained. As an illustration it is shown that effective superexchange interactions in the half-filled Hubbard model can be derived within the dual-ladder approximation.     

Density of states and zero Landau Level probed through capacitance of graphene

We report capacitors in which a finite electronic compressibility of graphene dominates the electrostatics, resulting in pronounced changes in capacitance as a function of magnetic field and carrier concentration. The capacitance measurements have allowed us to accurately map the density of states D, and compare it against theoretical predictions. Landau oscillations in D are robust and zero Landau level (LL) can easily be seen at room temperature in moderate fields. The broadening of LLs is strongly affected by charge inhomogeneity that leads to zero LL being broader than other levels.     

Spectral function of ferromagnetic 3d metals: a self-consistent LSDA+DMFT approach combined with the one-step model of photoemission

The electronic structure of ferromagnetic 3d-transition metals in the vicinity of the Fermi level is dominated by the spin-polarized d bands. Experimentally, this energy region can be probed in detail by means of angle-resolved ultraviolet photoemission and inverse photoemission. In several earlier studies the measured spectra were described either within a single-particle approach based on the local spin-density approximation including matrix-element effects within the so-called one-step model or by sophisticated many-body approaches neglecting these effects. In our analysis we combine for the first time correlation with matrix-element effects to achieve an improved interpretation of photoemission data from ferromagnetic nickel.