Quasiparticle Fluctuations Research Articles

Single-spin qubit magnetic spectroscopy of two-dimensional superconductivity

A single-spin qubit placed near the surface of a conductor acquires an additional contribution to its $1/T_1$ relaxation rate due to magnetic noise created by electric current fluctuations in the material. We analyze this technique as a wireless probe of superconductivity in atomically thin two dimensional materials. At temperatures $T \lesssim T_c$, the dominant contribution to the qubit relaxation rate is due to transverse electric current fluctuations arising from quasiparticle excitations. We demonstrate that this method enables detection of metal-to-superconductor transitions, as well as investigation of the symmetry of the superconducting gap function, through the noise scaling with temperature. We show that scaling of the noise with sample-probe distance provides a window into the non-local quasi-static conductivity of superconductors, both clean and disordered. At low temperatures the quasiparticle fluctuations get suppressed, yet the noise can be substantial due to resonant contributions from collective longitudinal modes, such as plasmons in monolayers and Josephson plasmons in bilayers. Potential experimental implications are discussed.

Strong reduction of quasiparticle fluctuations in a superconductor due to decoupling of the quasiparticle number and lifetime

We measure temperature dependent quasiparticle fluctuations in a small Al volume, embedded in a NbTiN superconducting microwave resonator. The resonator design allows for read-out close to equilibrium. By placing the Al film on a membrane, we enhance the fluctuation level and separate quasiparticle from phonon effects. When lowering the temperature, the recombination time saturates and the fluctuation level reduces a factor $\sim$100. From this we deduce that the number of free quasiparticles is still thermal. Therefore, the theoretical, inverse relation between quasiparticle number and recombination time is invalid in this experiment. This is consistent with quasiparticle trapping, where on-trap recombination limits the observed quasiparticle lifetime.

Entanglement entropy, quantum fluctuations, and thermal entropy in topological phases

Entanglement entropy in topologically ordered matter phases has been computed extensively using various methods. In this paper, we study the entanglement entropy of topological phases in two-spaces from a new perspective — the perspective of quasiparticle fluctuations. In this picture, the entanglement spectrum of a topologically ordered system encodes the quasiparticle fluctuations of the system, and the entanglement entropy measures the maximal quasiparticle fluctuations on the EB. As a consequence, entanglement entropy corresponds to the thermal entropy of the quasiparticles at infinite temperature on the entanglement boundary. We corroborates our results with explicit computation in the quantum double model with/without boundaries. We then systematically construct the reduced density matrices of the quantum double model on generic 2-surfaces with boundaries.

Full Numerical Simulations of Dynamical Response in Superconducting Single-Photon Detectors

We numerically study transport phenomena in a superconducting detector. Our approach is to simulate the three-dimensional time-dependent Ginzburg-Landau equation coupled with heat-diffusion and Maxwell equations. The simulation shows dynamical transition to a resistive state when an incident particle has energy higher than superconducting transition temperature. We show temporal behaviors of a superconducting gap function, depending on the incident energy scale. Furthermore, we discuss the applicability of our method to a superconducting single-photon detector. Focusing on the effects of coupling with heat sink and magnitude of quasi-particle fluctuations, we show a significant decrease of detector's threshold energy.

NOVEL MOTT-INSULATOR PHASES AND QUANTUM CRITICAL POINTS IN OPTICAL LATTICES

A self-consistent equation is derived for the order parameter of quantum phases in an optical lattice within a mean-field approximation to the Bose–Hubbard model. Analyzing the solutions of the self-consistent equation in terms of the number fluctuations of quasiparticles, the one-body density matrix of atoms, and the normalized correlation function of quasiparticles, we have identified two types of Mott-insulator phases that are characterized by different values of the order parameter, quasiparticle fluctuations, and correlations. We have also identified the quantum critical points separating these two types of Mott-insulator phases and the corresponding quantum critical lines on the phase diagram.

Quasiparticle number fluctuations in superconductors

We present a general theory of quasiparticle number fluctuations in superconductors. The theory uses the master equation formalism. First, we develop the theory for a single occupation variable. Although this simple system is insufficient to describe fluctuations in a physical superconductor, it is illustrative, allowing this discussion to serve as a self-contained introduction. We go on to develop a multivariate theory that allows for an arbitrary number of levels with transitions of arbitrary size between levels. We specialize the multivariate theory for two particular cases. First, we consider intrinsic quasiparticle fluctuations. In a previous Letter, these results were used to describe time-resolved measurements of thermodynamic fluctuations in a superconducting Al box [C.M. Wilson, L. Frunzio and D.E. Prober, Phys. Rev. Lett. 87, 067004 (2001)]. Finally, we extend these results to include fluctuations due to extrinsic loss processes.

Feshbach-resonance-induced atomic filamentation and quantum pair correlation in atom-laser-beam propagation.

We study the propagation of an atom laser beam through a spatial region with a magnetic field tuned around a Feshbach resonance. Magnetic fields below the resonance produce an effective focusing Kerr medium that causes a modulational instability of the atomic beam. Under appropriate circumstances, this results in beam breakup and filamentation seeded by quasiparticle fluctuations and in the generation of correlated atomic pairs.

Simulation of superconducting transitions in a 2D model system with local pairing

Monte Carlo simulations are used to provide a detailed comparison of the properties of the “hole-modulated hopping” model with experimentally determined properties of high- T c superconductors. New properties simulated in this work include the density of quasiparticle fluctuations above and below T c, the density of ordered quasiparticles and a temperature-dependent coherence length. Several comparisons are made between the simulated results and known properties of high- T c superconductors. Qualitative agreement is obtained, but quantitative discrepancies lead to the conclusion that the range of the pairing interaction in the hamiltonian used for this study is too short to account for the properties of real systems.

Noise mechanisms of high temperature superconductors as infrared detectors

Different noise mechanisms associated with the transport properties of high temperature superconductors are discussed. A theoretical model to predict the 1 f noise characteristics of high temperature superconductors to infrared excitations is proposed. Noise mechanisms are attributed to the effects of temperature variations on the quasiparticle fluctuations and kinetic characteristics of the superconductor. In the transition region the thermodynamic fluctuations modulate the conductivity of the sample causing voltage variations in the presnece of a bias current contributing to the 1 f noise spectral power. The predictions of the model are compared to recent experimental measurements on YBaCuO epitaxial and single crystal thin films. The theoretical predictions in both cases are in general smaller than the measured values. We attribute the descrapency to the effects of noise sources associated with the flux hopping process and thermally activated flux creep in the lattice structure which are not addressed in the current theory.

Effects of thermal and particle-number fluctuations on the giant isovector dipole modes for the58Ni nucleus in the finite-temperature random-phase approximation

Using the realistic single-particle energy spectrum obtained in the Woods-Saxon nucleon mean-field potential, the authors calculate the BCS pairing gap for 58Ni as a function of temperature taking into account the thermal and particle-number fluctuations. The strength distributions of the electric dipole transitions and the centroids of the isovector giant dipole resonance (IV-GDR) are computed in the framework of the finite-temperature random-phase approximation (RPA) based on the Hamiltonian of the quasiparticle-phonon nuclear model with separable dipole forces. It is shown that the change of the pairing gap at finite temperature can noticeably influence the IV-GDR localisation in realistic nuclei. By taking both thermal and quasiparticle fluctuations in the pairing gap into account the effect of the phase transition from superfluid to normal in the temperature dependence of the IV-GDR centroid is completely smeared out.

Logarithmic Rounding Above the Superconducting Transition in YBa2Cu3O7 Crystals

ABSTRACTThe temperature dependence of the electrical conductivity measured in the a-b basal plane of YB2Cu3O7 crystals displays a weak, nearly logarithmic divergence near Tc, which is weaker than the power-law functions predicted by superconducting fluctuations in 1, 2, or 3 dimensions. On the other hand, we find that quasi-particle fluctuations and pair-breaking in two-dimensions as given by Maki and Thompson, where the pair-breaking parameter δ is assumed to be proportional to temperature, gives by itself an excellent fit to the data.

Theory of the Low-Voltage Impedance of Superconductor-Insulator-Normal Metal Tunnel Junctions

A theory for the low-voltage impedance of a superconductor-insulator-normal metal tunnel junction is developed that includes the effects of charge imbalance and of quasiparticle fluctuations. A novel, inelastic, charge-imbalance relaxation process is identified that is associated with the junction itself. This new process leads to the surprising result that the charge-imbalance component of the dc resistance of a junction becomes independent of the electron-phonon scattering rate as the insulator resistance decreases.