Search Results (37 total)

Linearly polarized spectra of far-infrared (IR) transmission in HoMn₂O₅ multiferroic single crystals have been studied in the frequency range between 8.5 and 105 cm⁻¹ and for temperatures between 5 and 300 K. Polarization of IR-active excitations depends on the crystallographic directions in HoMn₂O₅ and is sensitive to the magnetic phase transitions. We attribute some of the infrared-active excitations to electric-dipole transitions between ligand-field (LF) split states of Ho³⁺ ions. For light polarization along crystalline b axis, the oscillator strength of electric dipoles at low frequencies (10.5, 13, and 18 cm⁻¹) changes significantly at the commensurate-incommensurate antiferromagnetic phase transition at T₃=19 K. This effect shows a strong correlation with the pronounced steps of the b-directional static dielectric function. We propose that the LF on Ho³⁺ connects the magnetism and dielectric properties of this compound through coupling with the Mn spin structure. We comment on the possibility for composite excitations of magnons and excited LF states.

The intriguing structural properties of a ZnMnGaO₄ film epitaxially grown on MgO (001) substrate have been investigated using synchrotron-radiation-based x-ray diffraction. The ZnMnGaO₄ film consisted of a self-assembled checkerboard (CB) structure with highly aligned and regularly spaced vertical nanorods. The lattice parameters of the orthorhombic and rotated tetragonal phases of the CB structure were analyzed by measuring H-K, H-L, and K-L cross-sectional reciprocal space maps. We demonstrate that symmetry of lattice distortions at the phase boundaries provides means for the coherent coexistence of two domain types within the film volume.

We investigate the interplay between the quantum coherence and statistics in electrically driven nanostructures. We obtain an expression for the admittance and the current noise for a driven nanocapacitor in terms of the Floquet scattering matrix and derive a nonequilibrium fluctuation-dissipation relation. As an interplay between the quantum phase coherence and the many-body correlation, the admittance has peak values whenever the noise power shows a step as a function of the nearby gate voltage. Our theory is demonstrated by calculating the admittance and noise of driven double-quantum dots.

The nature of the emergent phase near a putative quantum critical point in the bilayer ruthenate Sr₃Ru₂O₇ has been a recent subject of intensive research. It has been suggested that this phase may possess electronic nematic order (ENO). In this work, we investigate the possibility of nematic domain formation in the emergent phase, using a phenomenological model of electrons with ENO and its coupling to lattice degrees of freedom. The resistivity due to the scattering off the domain walls is shown to closely follow the ENO parameter. Our results provide qualitative explanations for the dependence of the resistivity on external magnetic fields in Sr₃Ru₂O₇.

We investigate the transport and the dynamical properties of tunnel-coupled double charge shuttles. The oscillation frequencies of two shuttles are mode locked to integer multiples of the applied voltage frequency ω. We show that left-right-symmetric double shuttles may generate direct net current due to bistable motions caused by parametric instability. The symmetry-broken direct current appears near ω=Ω₀/(2j-1), (j=1,2,…), where Ω₀ is the dressed resonance frequency of the relative motion of the two shuttles.

We have observed net electron transport at zero bias, induced by surface acoustic wave (SAW), through a double-walled carbon nanotube on top of a GaAs substrate. As gate voltage varied, the SAW-induced current showed quasiperiodic oscillations ranging from positive to negative values. Such zero-bias oscillations were found to be closely related with the Coulomb oscillation at finite bias. SAW-induced ratchet pumping could explain our observations.

Recent measurements show the superlattice shot noise Fano factor in a low field limit will not universally approach 1/3 as previously predicted. We theoretically investigate the superlattice shot noise behaviors based on the nonequilibrium Green’s function method. It is demonstrated that the shot noise reduction reflects the intrinsic nature of superlattice transport pictures. At low field limit, the Fano factor is reduced to a lower value by sweeping the superlattice from the sequential tunneling to miniband conduction regime via decreasing the barrier width. At high field, the superlattice is in the Wannier-Stark hopping regime and the shot noise will asymptotically approach the full Poisson value. These theoretically obtained superlattice shot noise behaviors show good qualitative agreement with the recent experimental observations.

We present transport properties of a strongly correlated quantum dot attached to two leads with a side coupled noninteracting quantum dot. Transport properties are analyzed using the slave boson mean field theory which is reliable in the zero temperature and low bias regime. It is found that the transport properties are determined by the interplay of two fundamental physical phenomena, i.e., the Kondo effects and the Fano interference. The linear conductance will depart from the unitary limit and the zero bias anomaly will be suppressed in the presence of interdot coupling. The zero bias shot noise Fano factor in mean field approximation varies with the interdot coupling and tends to the Poisson value. We find nonmonotonic increase of the shot noise Fano factor with the interdot coupling, which cannot be obtained within the noninteracting model.

Using a symmetry-based atomic scale theory of lattice distortions, we demonstrate that elastic textures, such as twin and antiphase boundaries, can generate intricate electronic heterogeneities in materials with strong electron-lattice coupling, as observed in perovskite manganites and other functional electronic materials.

We have observed coherent optical and acoustic phonon generation, which are strongly coupled to the charge-ordering (CO) transition in La_1−xCa_xMnO₃ (x=0.5, 0.58) using femtosecond optical pump-probe spectroscopy. Coherent optical phonons, observed at low temperatures, disappear above the charge-ordering temperature T_CO, while coherent acoustic phonons display the opposite behavior, disappearing gradually below T_CO. Coherent optical phonons are generated by the displacive excitation mechanism where their coupling to the photoexcited charge carriers is enhanced by the structural change corresponding to the CO phase transition. The oscillation frequency for the coherent acoustic phonon depends on the probe wavelength, which is consistent with the propagating strain pulse mechanism. The dramatic change of lattice constants across the charge-ordering transition explains the overall temperature dependence of the coherent acoustic phonon amplitude.

In the usual formulation of the weak localization(WL) effect, the mean free path of the conduction electron is assumed to be smaller than the geometric size of conductors. In multiwalled carbon nanotubes, however, the mean free path l is usually larger than its radius R. We consider ballistic correction to the usual WL conductance of multiwalled nanotubes within a semiclassical theory of disordered conductors. The ballistic correction to WL magnetoconductance is significant when the winding number of the associated interference paths is smaller than ∼l/(2πR). In this regime, ballistic paths along the circumference of the nanotube cause an additional correction δG∼G/(k_FR), where k_F is the Fermi wave vector. The ballistic corrections to the frequency-dependent conductance are significant when the frequency becomes comparable to the elastic scattering rate.

We present a detailed theoretical study of the ultrafast quasiparticle relaxation dynamics observed in normal metals and heavy-fermion materials with femtosecond time-resolved optical pump-probe spectroscopy. For normal metals, a nonthermal electron distribution gives rise to a temperature- (T) independent electron-phonon relaxation time at low temperatures, in contrast to the T^-3-divergent behavior predicted by the two-temperature model. For heavy-fermion compounds, we find that the blocking of electron-phonon scattering for heavy electrons within the density-of-states peak near the Fermi energy is crucial to explain the rapid increase of the electron-phonon relaxation time below the Kondo temperature. We propose the hypothesis that the slower Fermi velocity compared to the sound velocity provides a natural blocking mechanism due to energy and momentum-conservation laws.

We present an atomic scale theory of lattice distortions using strain-related variables and their constraint equations. Our approach connects constrained atomic length scale variations to continuum elasticity and describes elasticity at several length scales. We apply the approach to a two-dimensional square lattice with a monatomic basis and find the elastic deformations and hierarchical atomic relaxations in the vicinity of a domain wall between two different homogeneous strain states. We clarify the microscopic origin of gradient terms, some of which are included phenomenologically in Ginzburg-Landau theory, by showing that they are anisotropic.

Theoretical and computational results are presented clarifying the role of long-ranged strain interactions in determining the charge and orbital ordering in colossal magnetoresistance manganites. The strain energy contribution is found to be of order 20–30 meV/Mn and in particular stabilizes the anomalous “zigzag chain” order observed in many half-doped manganites.

The effects of long-range anisotropic elastic deformations on electronic structure in superconductors are analyzed within the framework of the Bogoliubov–de Gennes equations. Cases of twin boundaries and isolated defects are considered as illustrations. We find that the superconducting order parameter is depressed in the regions where pronounced lattice-deformation occurs. The calculated local density of states suggests that the electronic structure is strongly modulated in response to lattice deformations, and propagates to longer distances. In particular, this allows the trapping of low-lying quasiparticle states around defects. Some of our predictions can be directly tested by scanning tunneling microscopy experiments.

We present the first femtosecond studies of electron-phonon (e-ph) thermalization in heavy-fermion compounds. The e-ph thermalization time τ_ep increases below the Kondo temperature by more than 2 orders of magnitude as T=0  K is approached. Analysis using the two-temperature model and numerical simulations based on Boltzmann’s equations suggest that this anomalous slowing down of the e-ph thermalization derives from the large electronic specific heat and the suppression of scattering between heavy electrons and phonons.

We report the discovery of pressure-induced superconductivity in a semimetallic magnetic material CeTe_1.82. The superconducting transition temperature T_c=2.7 K (well below the magnetic ordering temperatures) under pressure (>2 kbar) is remarkably high, considering the relatively low carrier density due to a charge-density-wave transition associated with lattice modulation. The mixed magnetic structure of antiferromagnetism coexisting with ferromagnetism can provide a clue for this high T_c. We discuss a possible theoretical model for the superconducting pairing mechanism.

We theoretically investigate the ground-state properties of ferromagnetic metal/conjugated polymer interfaces. The work was partially motivated by recent experiments in which injection of spin-polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a half-metallic ferromagnetic colossal magnetoresistance (CMR) manganite that has explicit structural degrees of freedom. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. We find that there can be spin density polarization in the polymer near the interface. The spin-density oscillates and decays into the polymer with a decay length of about six times the lattice constant of the polymer. We find an expansion of the end bonds of the CMR manganite segment and a contraction of the polymer bonds near the interface. By adjusting the relative chemical potential of the contact and the polymer, electrons can be transferred into the polymer from the magnetic layer through the interfacial coupling. We calculate the density of states (DOS) before and after coupling for cases in which electrons are transferred and are not transferred to the polymer. The DOS has important consequences for spin injection under electrical bias: polarized spin injection is possible when the Fermi level of the ferromagnet lies below the the bipolaron level of the polymer. However, if the Fermi level of the CMR manganite lies above the bipolaron level of the polymer, the transferred electrons form bipolarons, which have no spin, and there is no spin density in the bulk of the polymer.

Dissipation of micro- and nanoscale mechanical structures is dominated by quantum-mechanical tunneling of two-level defects intrinsically present in the system. We find that at high frequencies—usually, for smaller, micron-scale structures—a novel mechanism of phonon pumping of two-level defects gives rise to weakly temperature-dependent internal friction, Q^-1, concomitant to the effects observed in recent experiments. Because of their size, comparable to or shorter than the emitted phonon wavelength, these structures suffer from superradiance-enhanced dissipation by the collective relaxation of a large number of two-level defects contained within the wavelength.

We investigate the energy spectra of clean incommensurate double-walled carbon nanotubes, and find that the overall spectral properties are described by the critical statistics similar to that known in the Anderson metal-insulator transition. In the energy spectra, there exist three different regimes characterized by Wigner-Dyson, Poisson, and semi-Poisson distributions. This feature implies that the electron transport in incommensurate multiwalled nanotubes can be either diffusive, ballistic, or intermediate between them, depending on the position of the Fermi energy.

The effects of uniaxial strain on the structural, orbital, optical, and magnetic properties of LaMnO₃ are calculated using realistic expressions for elastic energies, along with a tight-binding parametrization of the band theory and electron-phonon coupling. Tensile uniaxial strain of the order of 2% (i.e., of the order of magnitude of those induced in thin films by lattice mismatch with substrates) is found to change the magnetic ground state, causing dramatic changes in the band structure and optical conductivity spectrum. Related issues, including reasons why the observed (ππ0) orbital ordering is favored over a (πππ) periodicity and why the uniform tetragonal distortion mode is softer in insulating than in doped compounds, are discussed.

The nonlinear response to an external electric field is studied for classical noninteracting charged particles under the influence of a uniform magnetic field, a periodic potential, and an effective friction force. We find numerical and analytical evidence that the ratio of transverse to longitudinal resistance forms a Devil’s staircase. The staircase is attributed to the dynamical phenomenon of mode-locking.

The electron dephasing time τ_φ in a diffusive quantum dot is calculated by considering the interaction between the electron and dynamical defects, modeled as two level systems. Using the standard tunneling model of glasses, we obtain a linear temperature dependence of 1/τ_φ, consistent with the experimental observation. However, we find that, in order to obtain dephasing times on the order of nanoseconds, the number of two-level defects needs to be substantially larger than the typical concentration in glasses. We also find a finite system-size dependence of τ_φ, which can be used to probe the effectiveness of surface-aggregated defects.

The ground state and the dielectric response of stacked quantum rings are investigated in the presence of an applied magnetic field along the ring axis. For odd number of N of rings and an electric field perpendicular to the axis, a linear Stark effect occurs at distinct values of the magnetic field. At those fields energy levels cross in the absence of electric field. For even values of N a quadratic Stark effect is expected in all cases, but the induced electric polarization is discontinuous at those special magnetic fields. Experimental consequences for related nanostructures are discussed.