Narrow Results

Many of the functionally relevant collective vibrations of proteins and other biopolymers are expected to occur at terahertz frequencies. Precise absorption measurements combined with careful titration of biopolymers in water have allowed us to directly measure the terahertz absorption spectra associated with these motions, despite the strong background absorption of the solvent. We have also explored the circular dichroism spectroscopy of biomolecules over this same frequency range. Since circular dichroism requires the presence of net chirality in a molecule and chirality is present in nearly all biomaterial, it has the potential to capture the background free spectral features in biopolymers. To undertake these studies we have developed a broad band terahertz spectrometer suitable for both direct absorption and circular dichroism measurements of proteins in water between 0.75 – 3.72 THz. Direct terahertz absorption spectra of prototypical proteins bovine serum albumin (BSA) and hen egg white lysozyme have been documented and are described here. We have also successfully demonstrated the magnetic circular dichroism in semiconductors, and placed an upper bound on the terahertz circular dichroism signature of solvated BSA. In the terahertz frequency range, it appears that circular dichroism signatures are exceedingly small and detection remains a challenge.

We use a quasi-particle picture and the hard-loop approximation to study collective modes in anisotropic plasmas. We study a general class of anisotropic distribution functions and derive and solve the dispersion equations. We focus on imaginary modes because of their potential influence on plasma dynamics, and we study their dependence on the parameters that characterize the anisotropy of the system. We show that the strength of the imaginary modes is related to the oblateness of the distribution, and the size of the azimuthal Fourier coefficients.

In light of recent experiments, the study of the collective excitations in unconventional superconductors (USC) becomes very important. We build by path integration technique 2D and 3D models of p- and d-pairing for superfluids and superconductors (SC). Within these models we calculate the collective excitations in different USC (high temperature superconductors (HTSC), heavy fermion superconductors (HFSC) etc.) under p- and d-pairing. We considered both bulk and 2D systems. Some recent ideas concerning realization in HTSC of the mixtures of different states are investigated. In particular, we consider the mixture of d_x²-y² and d_xy states in HTSC. Obtained results could be used for interpretation of the sound and microwave absorption data as well as for identification of the type of pairing and order parameter in unconventional superconductors.

The derivation for collective modes of an interacting Bose gas trapped by an isotropic harmonic oscillator potential is presented using field-theoretic method. The presence of the two-body scattering term beyond the mean-field is seen to appear inevitably in the calculations, even in the simplest approximation. As a result we see the occurrence of a small number of non-condensate atoms in the ground state density fluctuations.

It is well known that most of the high temperature superconductors (at least hole-type) are in d-wave state. But it is still an unsolved problem whether it is a pure d-wave state or one has some kind of mixed state. Among the candidates for an admixture, there are s- and d-wave states. Existing experiments could not resolve this issue. New possibilities for experimental resolution of this problem are opened via recent observation of the collective modes in UBe₁₃ (heavy fermion superconductor) by microwave impedance technique experiments and in Sr₂RuO₄ (high temperature superconductor) by ultrasound attenuation experiments.

Some theoretical treatments show that the most likely state is a mixture of two d-wave states: d_x²-y² and d_xy with a small admixture of former state. To create the theoretical basis for investigation of possible mixed superconducting state in unconventional superconductors by sound attenuation and microwave absorption experiments, I derive for the first time a full set of equations for collective modes spectrum in d_x²-y²-state with small admixture of d_xy state. These equations allow to calculate the whole collective mode spectrum in mixed d_x²-y²+iεd_xy state and distinguish this state from pure d-wave states (whose collective mode spectrum has been calculated earlier) by ultrasound attenuation and microwave absorption experiments.

We propose a charge crystal model that captures all the essential physics of the high temperature superconductivity (HTS) in the long wavelength limit. Based on the recent transport and the far-infrared (far-IR) experiments, we argue that the three-dimensional (3D) ordering of the pinned two-dimensional (2D) square electronic lattice (EL) in each CuO₂ plane is the building block of HTS. Incorporating the physical picture derived from the neutron scattering experiments, we demonstrate that our model presents a coherent picture of the HTS. We suggest that the charge crystal model serves as a model for the microscopic theory and, hence, offers the key to the mechanism for the HTS.

Electron spin resonance (ESR) in exchanged-coupled paramagnets is a probe of field-dependent collective modes in a strongly coupled many-body system. We utilize a previously developed approach for the analysis of collective modes in such systems to express the experimentally determined g-factors of the ESR modes in an anisotropic magnet in terms of the microscopic (isolated spin) g-factors and the ratios of the static susceptibilities along the principal directions. Applications of the theory to the ferromagnetic insulator CrBr₃ and the Kondo lattice systems YbRh₂Si₂, YbCo₂Si₂, YbIr₂Si₂ and CeRuPO are discussed.

We study the weakly damped electromagnetic waves in strongly anisotropic organic conductors with the chain-like character of the intermolecular overlapping at various orientations of an external magnetic field and wave vector. We obtain the analytical expressions for the spectra of the collective modes in a number of limiting cases, in particular, the dispersion relations in a short-wavelength limit when the eigen frequencies are not close to the resonance frequency and its harmonics. The numerical analysis giving full enough description of the dispersion of the weakly damped eigenmodes is presented. We find the spectrum of the eigenmodes owing to electron oscillations along the least conductivity direction. We show that if the orientation of the wave vector k is not too close to the least conductivity direction, the dependence of the eigen frequencies on k has pronounced oscillatory character.

We present an analytical study for density profiles of two-component Bose–Einstein condensates (BECs) in an isotropic trap within the miscible regime. The analytical equations for the density profiles and the conditions for three different density configurations are presented, which allow us to explore the density profile diagrams of general two-component BECs depending on broad ranges of experimental parameters. Furthermore, we show that the obtained explicit expressions of the density profiles can also be applied to study the collective modes of the two-component BECs, and the frequencies of the two-component BECs clearly characterize the density configurations.

The neutron star inner crust is assumed to be superfluid at relevant temperatures. The contribution of neutron quasiparticles to thermodynamic and transport properties of the crust is therefore strongly suppressed by the pairing gap. Nevertheless, the neutron gas still has low-energy excitations, namely long-wavelength collective modes. We summarize different approaches to describe the collective modes in the crystalline phases of the inner crust and present an improved model for the description of the collective modes in the pasta phases within superfluid hydrodynamics.

In this work, we calculate the level density parameter for several nuclei by deploying the semiclassical trace formula for spherically symmetric harmonic oscillator potential with spin-orbit interactions, which describes the quantum shell effects in terms of the classical periodic orbits. With the usage of recently proposed Laplace like formula of level density parameter by Canbula et al., the effects of collective rotations and vibrations as well as deformations in nuclei are incorporated. The comparison of our results with the experimental ones show a good agreement. Later on, the study is also extended to the calculation of nuclear level density and survival probability in several superheavy nuclei (SHN).

Low-frequency collective K^π = 0⁺ modes in deformed neutron-rich unstable nuclei are investigated by means of the quasiparticle-random-phase approximation (QRPA) based on the coordinate space Hartree-Fock-Bogoliubov (HFB) formalism. It is found that the spatially extended structure of neutron quasiparticle wave-functions brings about a striking enhancement of the transition strengths. It is also found that the quadrupole pairing correlation plays an important role in generating coherence among two-quasiparticle excitations of neutrons.

The conditions under which nuclear collective modes couple to plasmon modes in asymmetric nuclear matter (ANM) neutralized by electrons, which is of interest for the study of neutron stars and supernovae, are investigated. We take a mean-field approach to nuclear matter, and the Coulomb field is included. We show that the coupling may be so strong that it affects the onset of the nuclear mode and it may also change its isovector/isoscalar character.

Many of the functionally relevant collective vibrations of proteins and other biopolymers are expected to occur at terahertz frequencies. Precise absorption measurements combined with careful titration of biopolymers in water have allowed us to directly measure the terahertz absorption spectra associated with these motions, despite the strong background absorption of the solvent. We have also explored the circular dichroism spectroscopy of biomolecules over this same frequency range. Since circular dichroism requires the presence of net chirality in a molecule and chirality is present in nearly all biomaterial, it has the potential to capture the background free spectral features in biopolymers. To undertake these studies we have developed a broad band terahertz spectrometer suitable for both direct absorption and circular dichroism measurements of proteins in water between 0.75 – 3.72 THz. Direct terahertz absorption spectra of prototypical proteins bovine serum albumin (BSA) and hen egg white lysozyme have been documented and are described here. We have also successfully demonstrated the magnetic circular dichroism in semiconductors, and placed an upper bound on the terahertz circular dichroism signature of solvated BSA. In the terahertz frequency range, it appears that circular dichroism signatures are exceedingly small and detection remains a challenge.

The derivation for collective modes of an interacting Bose gas trapped by an isotropic harmonic oscillator potential is presented using field-theoretic method. The presence of the two-body scattering term beyond the mean-field is seen to appear inevitably in the calculations, even in the simplest approximation. As a result we see the occurrence of a small number of non-condensate atoms in the ground state density fluctuations.

A finite rank separable approximation for Skyrme interactions is applied to study the collective nuclear modes in stable and unstable nuclei. The coupling between one- and two-phonon terms in the wave functions are taken into account. Some possibilities for experimental investigations of the manifestations of the pseudospin symmetry in the spectra of odd nuclei with Z ≥ 100 are indicated.