Search Results (83 total)

We consider ultracold matter of spin-two atoms in optical lattices. We derive an effective Hamiltonian for the studies of spin ordering in Mott states and investigate hyperfine spin correlations. Particularly, we diagonalize the Hamiltonian in an on-site Hilbert space taking into account spin-dependent interactions and exchange between different sites. We obtain phase diagrams and quantum phase transitions between various magnetic phases.

The electronic structure of lanthanide and actinide compounds is often characterized by orbital ordering of localized f electrons. Density-functional theory studies of such systems using the currently available local-density approximation (LDA)+U method are plagued by significant orbital-dependent self-interaction, leading to erroneous orbital ground states. An alternative scheme that modifies the exchange, not Hartree, energy is proposed as a remedy. We show that our LDA+U approach reproduces the expected degeneracy of f¹ and certain f² states in free ions and the correct ground states in solid PrO₂. We expect our method to be useful in studying electronic excitations and entropies in f and heavy-d elements.

We use neutron scattering to study the effect of electron doping on the structural or magnetic order in BaFe₂As₂. In the undoped state, BaFe₂As₂ exhibits simultaneous structural and magnetic phase transitions below 143 K. Upon electron doping to form BaFe_1.96Ni_0.04As₂, the system first displays the lattice distortion near ∼97  K, and then orders antiferromagnetically at 91 K before developing weak superconductivity below ∼15  K. The effect of electron doping is to reduce the c-axis exchange coupling in BaFe₂As₂ and induce quasi-two-dimensional (2D) spin excitations. These results suggest that the transition from 3D spin waves to quasi-2D spin excitations by electron doping is important for the separated structural and magnetic phase transitions in iron arsenides.

In this Letter, we study bosonic atoms at large scattering lengths using a variational method where the condensate amplitude is a variational parameter. We further examine momentum distribution functions, chemical potentials, the speed of sound, and spatial density profiles of cold bosonic atoms in a trap in this limit. The latter two properties turn out to bear similarities to those of Fermi gases. The estimates obtained here are applicable near Feshbach resonances, particularly when the fraction of atoms forming three-body structures is small and can be tested in future cold atom experiments.

Using first-principles calculations, we study the effect of cation substitution on the transition-metal sublattice in phospho-olivines, with special attention given to the Li_x(Fe_1−yMn_y)PO₄ system. We use a cluster expansion model derived from first-principles with Monte Carlo simulations to calculate finite-T phase diagrams, voltage curves, and solubility limits of the system. The phase diagram of Li_x(Fe_1−yMn_y)PO₄ shows two low-temperature miscibility gaps separated by a solid solution phase centered at Li composition x≈y, which corresponds to a state where most Fe ions are oxidized and most Mn are not. This intermediate low-T solid solution is stabilized by the dilution of phase-separating interactions caused by the disorder of redox potentials on the transition-metal sites. The calculated voltage curves show two plateaus at ∼4–4.2 V and ∼3.5–3.7 V, corresponding to the Mn³⁺/Mn²⁺ and Fe³⁺/Fe²⁺ redox couples, respectively, with an extended sloping region in between corresponding to the low-T solid solution phase. In agreement with experiment, we find that the Mn³⁺/Mn²⁺ (Fe³⁺/Fe²⁺) voltage is decreased (increased) by Fe (Mn) substitution. We explain this by considering the energy of the solid solution which is the discharged (charged) state for these redox couples and argue that such changes are generic to all mixed olivine systems. We also find reduced phase transformation polarization on both plateaus which we attribute to the decreased composition difference between the oxidized and reduced state for each redox couple

Physical systems must fulfill a number of conditions to qualify as useful quantum bits (qubits) for quantum-information processing, including ease of manipulation, long decoherence times, and high fidelity readout operations. Since these conditions are hard to satisfy with a single system, it may be necessary to combine different degrees of freedom. Here we discuss a possible system based on electronic and nuclear spin degrees of freedom in trapped ions. The nuclear spin yields long decoherence times, while the electronic spin, in a magnetic field gradient, provides efficient manipulation, and the optical transitions of the ions assure a selective and efficient initialization and readout.

We have studied the quantum spin dynamics of small condensates of cold sodium atoms. For a condensate initially prepared in a mean-field ground state, we show that coherent spin dynamics is purely driven by quantum fluctuations of collective spin coordinates and can be tuned by quadratic Zeeman coupling and magnetization. This dynamics in small condensates can be probed in a high-finesse optical cavity where the temporal behaviors of the excitation spectra of a coupled condensate-photon system reveal the time evolution of populations of atoms at different hyperfine spin states.

We have investigated the dispersion renormalization Z_disp in La_2−xSr_xCuO₄ over the wide doping range of x=0.03–0.30, for binding energies extending to several hundred meV’s. Strong correlation effects conspire in such a way that the system exhibits a local-density-approximation-like dispersion which essentially “undresses” (Z_disp→1) as the Mott insulator is approached. Our finding that the Mott insulator contains “nascent” or “preformed” metallic states with a vanishing spectral weight offers a challenge to existing theoretical scenarios for cuprates.

The electronic properties of a particular class of domain walls in gapped graphene are investigated. We show that they can support midgap states which are localized in the vicinity of the domain wall and propagate along its length. With a finite density of domain walls, these states can alter the electronic properties of gapped graphene significantly. If the midgap band is partially filled, the domain wall can behave like a one-dimensional metal embedded in a semiconductor and could potentially be used as a single-channel quantum wire.

We investigate the dynamic creation of fractionalized half-quantum vortices in Bose-Einstein condensates of sodium atoms. Our simulations show that both individual half-quantum vortices and vortex lattices can be created in rotating optical traps when additional pulsed magnetic trapping potentials are applied. We also find that a distinct periodically modulated spin-density-wave spatial structure is always embedded in square half-quantum vortex lattices. This structure can be conveniently probed by taking absorption images of ballistically expanding cold atoms in a Stern-Gerlach field.

Temporal evolution of a macroscopic condensate of ultracold atoms is usually driven by mean-field potentials, either due to scattering between atoms or due to coupling to external fields; and coherent quantum dynamics of this type have been observed in various cold atom experiments. In this paper, we report results of studies of a class of quantum spin dynamics which are purely driven by zero point quantum fluctuations of spin collective coordinates. Unlike the usual mean-field coherent dynamics, quantum-fluctuation-controlled spin dynamics (or QFCSD) studied here are very sensitive to variation of quantum fluctuations and the corresponding driving potentials induced by zero point motions can be tuned by four to five orders of magnitude using optical lattices. These dynamics have unique dependence on optical lattice potential depths and quadratic Zeeman fields. We also find that thermal fluctuations generally can further enhance the induced potentials although the enhancement in deep optical lattices is much less substantial than in traps or shallow lattices. QFCSD can be potentially used to calibrate quantum fluctuations and investigate correlated fluctuations and various universal scaling properties near quantum critical points.

Macroscopic strain was hitherto considered a necessary corollary of deformation twinning in coarse-grained metals. Recently, twinning has been found to be a preeminent deformation mechanism in nanocrystalline face-centered-cubic (fcc) metals with medium-to-high stacking fault energies. Here we report a surprising discovery that the vast majority of deformation twins in nanocrystalline Al, Ni, and Cu, contrary to popular belief, yield zero net macroscopic strain. We propose a new twinning mechanism, random activation of partials, to explain this unusual phenomenon. The random activation of partials mechanism appears to be the most plausible mechanism and may be unique to nanocrystalline fcc metals with implications for their deformation behavior and mechanical properties.

Using first-principles calculations within the generalized gradient approximation (GGA)+U framework, we investigate several surface properties of olivine structure LiFePO₄. Calculated surface energies and surface redox potentials are found to be very anisotropic. Low-energy surfaces are in the [1 0 0], [0 1 0], [0 1 1], [1 0 1], and [2 0 1] orientations of the orthorhombic structure. We find that the coordination loss of Fe atoms on the surface is energetically more unfavorable than for Li, and generally a low-energy surface has fewer Fe-O bonds affected by the surface cut. Conversely, undercoordinated Li on the surface are somehow beneficial to reduce the energy of a surface except for the twofold coordinated Li. With the calculated surface energies, we provide the thermodynamic equilibrium shape of the LiFePO₄ crystal through a Wulff construction. The two low-energy surfaces (0 1 0) and (2 0 1) dominate in the Wulff shape and make up almost 85% of the surface area. Similar calculations for FePO₄ indicate a very low energy for the (0 1 0) surface of FePO₄. This result suggests that surface chemistry can induce a change in the aspect ratio of the Wulff shape. Surface redox potentials for the extraction and insertion of Li from various surfaces are also investigated in this work. The Li redox potential for the (0 1 0) surface is calculated to be 2.95  V, which is significantly lower than the bulk value of 3.55  V. For several other surfaces the Li extraction potential is above the bulk potential. We also develop a simple model that can be used to predict surface energies based on the change in the coordination of Fe and Li.

To date, angle-resolved photoemission spectroscopy has been successful in identifying energy scales of the many-body interactions in correlated materials, focused on binding energies of up to a few hundred meV below the Fermi energy. Here, at higher-energy scale, we present improved experimental data from four families of high-T_c superconductors over a wide doping range that reveal a hierarchy of many-body interaction scales focused on: the low-energy anomaly (“kink”) of 0.03–0.09 eV, a high-energy anomaly of 0.3–0.5 eV, and an anomalous enhancement of the width of the local-density-approximation-based CuO₂ band extending to energies of ≈2 eV. Besides their universal behavior over the families, we find that all of these three dispersion anomalies also show clear doping dependence over the doping range presented.

It is shown that zero point quantum fluctuations completely lift the accidental continuous degeneracy that is found in mean field analysis of quantum spin nematic phases of hyperfine spin-2 cold atoms. The result is two distinct ground states which have higher symmetries: a uniaxial spin nematic and a biaxial spin nematic with dihedral symmetry Dih₄. There is a novel first-order quantum phase transition between the two phases as atomic scattering lengths are varied. We find that the ground state of ⁸⁷Rb atoms should be a uniaxial spin nematic. We note that the energy barrier between the phases could be observable in dynamical experiments.

In this Letter we study discrete symmetries of mean field manifolds of condensates of F=2 cold atoms, and various unconventional quantum vortices. Discrete quaternion symmetries result in two species of spin defects that can only appear in integer vortices while cyclic symmetries are found to result in a phase shift of 2π/3 (or 4π/3) and therefore 1/3– (or 2/3–) quantum vortices in condensates. We also briefly discuss 1/3–quantum vortices in condensates of trimers.

Space charge and coherent synchrotron radiation may deteriorate electron beam quality when the beam passes through a magnetic bunch compressor. This paper presents the transverse phase-space tomographic measurements for a compressed beam at 60 MeV, around which energy the first stage of magnetic bunch compression takes place in most advanced linacs. Transverse phase-space bifurcation of a compressed beam is observed at that energy, but the degree of the space charge-induced bifurcation is appreciably lower than the one observed at 12 MeV.

In this Letter we study various spin correlated insulating states of F=2 cold atoms in optical lattices. We find that the effective spin exchange interaction due to virtual hopping contains an octopole coupling between two neighboring lattice sites. Depending on scattering lengths and numbers of particles per site the ground states are either rotationally invariant dimer or trimer Mott insulators or insulating states with various spin orders. Three spin-ordered insulating phases are ferromagnetic, cyclic, and nematic Mott insulators. We estimate the phase boundaries for states with different numbers of atoms per lattice site.

We demonstrate that configurational electronic entropy, previously neglected, in ab initio thermodynamics of materials can qualitatively modify the finite-temperature phase stability of mixed-valence oxides. While transformations from low-T ordered or immiscible states are almost always driven by configurational disorder (i.e., random occupation of lattice sites by multiple species), in FePO₄-LiFePO₄ the formation of a solid solution is almost entirely driven by electronic rather than ionic configurational entropy. We argue that such an electronic entropic mechanism may be relevant to most other mixed-valence systems.

In this paper, we illustrate the possible cyclic fermion pairing states across Feshbach resonances in optical lattices. In cyclic fermion pairing, the pairing amplitude exhibits an oscillatory behavior as the detuning varies. We estimate the quasiparticle gaps in different regimes of the resonances.

Using first-principles pseudopotential calculations, we investigate the formation and transport of small polarons in olivine Li_xFePO₄. It is demonstrated that excess charge carriers form small polarons in LiFePO₄ and FePO₄. Lower limits to the activation barrier for small polaron migration are calculated within the GGA+U framework. Additionally, the interaction between lithium ions and polarons is investigated and estimates of binding energies between lithium ions and polarons are provided. Our results show that the binding energy between electron polarons and Li⁺ ions in FePO₄ is lower than that between hole polarons and lithium vacancies in LiFePO₄. The electron transfer rate is predicted to be higher in FePO₄ than in LiFePO₄.

A free relativistic electron in an electromagnetic field is a pure case of a light-matter interaction. In the laboratory environment, this interaction can be realized by colliding laser pulses with electron beams produced from particle accelerators. The process of single photon absorption and reemission by the electron, so-called linear Thomson scattering, results in radiation that is Doppler shifted into the x-ray and γ-ray regions. At elevated laser intensity, nonlinear effects should come into play when the transverse motion of the electrons induced by the laser beam is relativistic. In the present experiment, we achieved this condition and characterized the second harmonic of Thomson x-ray scattering using the counterpropagation of a 60 MeV electron beam and a subterawatt CO₂ laser beam.

I argue that certain bosonic insulator-superfluid phase transitions as an interaction constant varies are driven by emergent geometric properties of insulating states. I examine the renormalized chemical potential and population of disordered bosons at different energy levels. These quantities define the geometric aspect of an effective low energy Hamiltonian which I employ to investigate various resonating states and quantum phase transitions. In a mean field approximation, I also demonstrate that the quantum phase transitions are in the universality class of a percolation problem.

The author has studied the influence of fermion-boson conversion on Mott states near Feshbach resonances. It is demonstrated that Mott states are unstable with respect to fermion-boson conversion. A branch of collective modes in superfluids has been found, which involve antisymmetric phase oscillations in fermionic and bosonic channels and are always gapped. The low-energy quantum dynamics of a Fermi-Bose superfluid can be fully characterized by either an effective coupled U(1)⊗U(1) quantum rotor Hamiltonian or a coupled XXZ⊗XXZ spin model.

We used point-contact tunneling spectroscopy to study the superconducting pairing symmetry of electron-doped Nd_1.85Ce_0.15CuO_4−y (NCCO) and hole-doped La_1.89Sr_0.11CuO₄ (LSCO). Nearly identical spectra without zero bias conductance peak (ZBCP) were obtained on the (110) and (100) oriented surfaces (the so-called nodal and anti-nodal directions) of NCCO. In contrast, LSCO showed a remarkable ZBCP in the nodal direction as expected from a d-wave superconductor. Detailed analysis reveals an s-wave component in the pairing symmetry of the NCCO sample with Δ∕k_BT_c=1.66, a value remarkably close to that of a weakly coupled Bardeen-Cooper-Schriffer (BCS) superconductor. We argue that this s-wave component is formed at the Fermi surface pockets centered at (±π,0) and (0,±π) although a d-wave component may also exist.