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Finding the Elusive Sliding Phase in the Superfluid-Normal Phase Transition Smeared by -Axis Disorder
We consider a stack of weakly Josephson coupled superfluid layers with -axis disorder in the form of random superfluid stiffnesses and vortex fugacities in each layer as well as random interlayer coupling strengths. In the absence of disorder this system has a 3D type superfluid-normal phase transition as a function of temperature. We develop a functional renormalization group to treat the effects of disorder, and demonstrate that the disorder results in the smearing of the superfluid-normal phase transition via the formation of a Griffiths phase. Remarkably, in the Griffiths phase, the emergent power-law distribution of the interlayer couplings gives rise to a sliding Griffiths superfluid, with a finite stiffness in the direction along the layers, and a vanishing stiffness perpendicular to it.Physic
Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps
We investigate the equilibrium and dynamical properties of the Bose-Hubbard
model and the related particle-hole symmetric spin-1 model in the vicinity of
the superfluid to Mott insulator quantum phase transition. We employ the
following methods: exact-diagonalization, mean field (Gutzwiller), cluster
mean-field, and mean-field plus Gaussian fluctuations. In the first part of the
paper we benchmark the four methods by analyzing the equilibrium problem and
give numerical estimates for observables such as the density of double
occupancies and their correlation function. In the second part, we study
parametric ramps from the superfluid to the Mott insulator and map out the
crossover from the regime of fast ramps, which is dominated by local physics,
to the regime of slow ramps with a characteristic universal power law scaling,
which is dominated by long wavelength excitations. We calculate values of
several relevant physical observables, characteristic time scales, and an
optimal protocol needed for observing universal scaling.Comment: 23 pages, 13 figure
Relaxation of Fermionic Excitations in a Strongly Attractive Fermi Gas in an Optical Lattice
We theoretically study the relaxation of high energy single particle
excitations into molecules in a system of attractive fermions in an optical
lattice, both in the superfluid and the normal phase. In a system characterized
by an interaction scale and a tunneling rate , we show that the
relaxation rate scales as in the large
limit. We obtain explicit expressions for the exponent , both in the
low temperature superfluid phase and the high temperature phase with pairing
but no coherence between the molecules. We find that the relaxation rate
decreases both with temperature and deviation of the fermion density from
half-filling. We show that quasiparticle and phase degrees of freedom are
effectively decoupled within experimental timescales allowing for observation
of ordered states even at high total energy of the system.Comment: 5 pages, 3 figure
Competition between pairing and ferromagnetic instabilities in ultracold Fermi gases near Feshbach resonances
We study the quench dynamics of a two-component ultracold Fermi gas from the
weak into the strong interaction regime, where the short time dynamics are
governed by the exponential growth rate of unstable collective modes. We obtain
an effective interaction that takes into account both Pauli blocking and the
energy dependence of the scattering amplitude near a Feshbach resonance. Using
this interaction we analyze the competing instabilities towards Stoner
ferromagnetism and pairing.Comment: 4+epsilon pages, 4 figure
Charge transfer excitons in optical absorption spectra of C60-dimers and polymers
Charge-transfer (CT) exciton effects are investigated for the optical
absorption spectra of crosslinked C60 systems by using the intermediate exciton
theory. We consider the C60-dimers, and the two (and three) molecule systems of
the C60-polymers. We use a tight-binding model with long-range Coulomb
interactions among electrons, and the model is treated by the Hartree-Fock
approximation followed by the single-excitation configuration interaction
method. We discuss the variations in the optical spectra by changing the
conjugation parameter between molecules. We find that the total CT-component
increases in smaller conjugations, and saturates at the intermediate
conjugations. It decreases in the large conjugations. We also find that the
CT-components of the doped systems are smaller than those of the neutral
systems, indicating that the electron-hole distance becomes shorter in the
doped C60-polymers.Comment: Figures should be requested to the autho
Proposal for Coherent Coupling of Majorana Zero Modes and Superconducting Qubits Using the 4π Josephson Effect
We propose to use an ancilla fluxonium qubit to interact with a Majorana qubit hosted by a topological one-dimensional wire. The coupling is obtained using the Majorana qubit-controlled 4π Josephson effect to flux bias the fluxonium qubit. We demonstrate how this coupling can be used to sensitively identify topological superconductivity, to measure the state of the Majorana qubit, to construct 2-qubit operations, and to implement quantum memories with topological protection.Physic
Magnetic field enhancement of superconductivity in ultra-narrow wires
We study the effect of an applied magnetic field on sub-10nm wide MoGe and Nb
superconducting wires. We find that magnetic fields can enhance the critical
supercurrent at low temperatures, and does so more strongly for narrower wires.
We conjecture that magnetic moments are present, but their pair-breaking
effect, active at lower magnetic fields, is suppressed by higher fields. The
corresponding microscopic theory, which we have developed, quantitatively
explains all experimental observations, and suggests that magnetic moments have
formed on the wire surfaces.Comment: 4 pages, 3 figures, 1 tabl
Structure and properties of the stable two-dimensional conducting polymer Mg5C60
We present a study on the structural, spectroscopic, conducting,
and
magnetic properties of Mg5C60, which is a two-dimensional (2D)
fulleride polymer. The polymer phase is stable up to the
exceptionally
high temperature of 823 K. The infrared and Raman studies
suggest the
formation of single bonds between the fulleride ions and
possibly
Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient
temperature, as
shown by electron spin resonance and microwave conductivity
measurements. The smooth transition from a metallic to a
paramagnetic
insulator state below 200 K is attributed to Anderson
localization
driven by structural disorder
Lifetime of double occupancies in the Fermi-Hubbard model
We investigate the decay of artificially created double occupancies in a
repulsive Fermi-Hubbard system in the strongly interacting limit using
diagrammatic many-body theory and experiments with ultracold fermions on
optical lattices. The lifetime of the doublons is found to scale exponentially
with the ratio of the on-site repulsion to the bandwidth. We show that the
dominant decay process in presence of background holes is the excitation of a
large number of particle hole pairs to absorb the energy of the doublon. We
also show that the strongly interacting nature of the background state is
crucial in obtaining the correct estimate of the doublon lifetime in these
systems. The theoretical estimates and the experimental data are in fair
quantitative agreement
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