Number Of Quasiparticles Research Articles

Rotation, pairing and blocking in high-K isomers explored through the rotational g-factor

The paired superfluid system of nucleons in nuclei, and their excitations, present properties of general physics interest. Collective magnetic g-factors gR, obtained from a novel analysis of data for multi-quasi-particle configurations in high- Kisomers, are shown to exhibit a striking systematic variation with the relative number of proton and neutron quasi-particles, Np − Nn. Basic ideas of pairing, its quenching by quasi-particle excitation and the consequent changes to moment of inertia and collective magnetism are discussed. Existing model calculations do not reproduce the observed gR variation adequately. The new findings open a fresh window on these effects and raise the important question of just which nucleons contribute to the ‘collective’ properties of these nuclei.

Parity independence of the zero-bias conductance peak in a nanowire based topological superconductor-quantum dot hybrid device.

We explore the signatures of Majorana fermions in a nanowire based topological superconductor-quantum dot-topological superconductor hybrid device by charge transport measurements. At zero magnetic field, well-defined Coulomb diamonds and the Kondo effect are observed. Under the application of a finite, sufficiently strong magnetic field, a zero-bias conductance peak structure is observed. It is found that the zero-bias conductance peak is present in many consecutive Coulomb diamonds, irrespective of the even-odd parity of the quasi-particle occupation number in the quantum dot. In addition, we find that the zero-bias conductance peak is in most cases accompanied by two differential conductance peaks, forming a triple-peak structure, and the separation between the two side peaks in bias voltage shows oscillations closely correlated to the background Coulomb conductance oscillations of the device. The observed zero-bias conductance peak and the associated triple-peak structure are in line with Majorana fermion physics in such a hybrid topological system.

Self-organization in foliated phase space: Construction of a scale hierarchy by adiabatic invariants of magnetized particles

Adiabatic invariants foliate phase space, and impart a macro-scale hierarchy by separating microscopic variables. On a macroscopic leaf, long-scale ordered structures are created while maximizing entropy. A plasma confined in a magnetosphere is invoked for unveiling the organizing principle ---in the vicinity of a magnetic dipole, the plasma self-organizes to a state with a steep density gradient. The resulting nontrivial structure has maximum entropy in an appropriate, constrained phase space. One could view such a phase space as a leaf foliated in terms of Casimir invariants ---adiabatic invariants measuring the number of quasi-particles (macroscopic representation of periodic motions) are identified as the relevant Casimir invariants. The density clump is created in response to the inhomogeneity of the energy level (frequency) of quasi-particles.

Correlation between polarity of magnetoresistance ratio and tunnel resistance in ferromagnetic single-electron transistor with superconductive island

We report on the correlation between the polarity of magnetoresitance ratios and the tunnel resistance observed in ferromagnetic single-electron transistors with a superconductive island (FM–SC–FM SETs). Negative magnetoresistance ratios are observed at approximately the superconductive gap voltages, which indicates that an excessive number of spin-polarized quasiparticles accumulated in a superconductive island suppress the superconductive gaps. Since the spin polarization in a superconductor decays with time, the spin injection rate should be high enough to observe the suppression of superconductive gaps. The spin injection rate at a certain bias voltage is inversely proportional to the tunnel resistance; hence, the polarity of magnetoresistance ratio should correlate with the tunnel resistance. We fabricated 26 FM–SC–FM SETs. Samples having tunnel resistances lower than 100 kΩ exhibited negative magnetoresistance ratios, while samples having tunnel resistances higher than 600 kΩ exhibited positive magnetoresistance ratios.

Parity Measurement in Topological Josephson Junctions

We study the properties of a topological Josephson junction made of both edges of a two-dimensional topological insulator. We show that, due to fermion parity pumping across the bulk, the global parity of the junction has a clear signature in the periodicity and critical value of the Josephson current. In particular, we find that the periodicity with the flux changes from 4π in a junction with an even number of quasiparticles to 2π in the odd sector. In the case of long junctions, we exhibit a rigorous mathematical connection between the spectrum of Andreev bound states and the fermion parity anomaly, through bosonization. Additionally, we discuss the rather quantitative effects of Coulomb interactions on the Josephson current.

Photon-Noise Limited Performance in Aluminum LEKIDs

We have measured noise in aluminum lumped element kinetic inductance detectors (LEKIDs) in dark conditions at different base temperatures and with optical illumination from a variable temperature blackbody source. LEKIDs are photon-sensitive superconducting resonators coupled to planar transmission lines. We convert variations in the raw in-phase (eI) and quadrature (eQ) signals from a fixed frequency source transmitted through a transmission line coupled to the LEKID into a measure of the fluctuation in the resonant frequency of the LEKID (ef) using the measured electrical response of the resonator to a swept frequency source. We find that the noise of the LEKID in the dark has a constant frequency fluctuation level, e f0 which is rolled off at a base temperature-dependent frequency corresponding to the quasiparticle lifetime in the device. Above this frequency, the noise is dominated by amplifier noise at a level a factor of 2-10 times lower than the low frequency white noise level depending on the quality factor of the resonator. The amplitude of this noise and the frequency cutoff agree well with the expected frequency flucution level from generation and recombination of thermal quasiparticles from a simple Mattis-Bardeen model. When we illuminate the device with a variable temperature blackbody source through a bandpass filter centered at a frequency of 150 GHz, we observe a reduction in the quasiparticle lifetime and an increase in the level of frequency fluctuation noise as the blackbody temperature is increased. This indicates that the quasiparticle number is dominated by optically generated quasiparticles and that the noise in the device is dominated by photon noise.

Crossed Andreev reflections in superconductor and fractional quantum Hall liquids hybrid system

The tunnel junction of an s-wave superconductor (SC) coupled to two spatially separated fractional quantum Hall (FQH) liquids with opposite spin polarizations is investigated theoretically. In the strong coupling limit, crossed Andreev reflection occurs, where a set of quasiparticles reflected as quasiholes on each FQH liquid side with Cooper pairs transmitted into the SC. The numbers of quasiparticles and quasiholes involved in the AR process satisfy a specific selection rule. In addition, such a setup can serve as an entangler, creating a spin singlet of two electrons, which are spatially separated.

Time-dependent equation for the magnetic order parameter near the quantum critical point in multiband superconductors with a spin-density wave

Using a simple two-band model for Fe-based pnictides and the generalized Eilenberger equation, we present a microscopic derivation of a time-dependent equation for the amplitude of the spin density wave near the quantum critical point where it turns to zero. This equation describes the dynamics of the magnetic---$m$, as well as the superconducting order parameter---$\Delta$. It is valid at low temperatures $T$ and small $m$ (${T, m \ll \Delta}$) in a region of coexistence of both order parameters, $m$ and $\Delta$. The boundary of this region is found in the space of the nesting parameter $\{\mu_{0},\mu_{\phi}\}$ where $\mu_{0}$ describes the relative position of the electron and the hole pockets on the energy scale, and $\mu_{\phi}$ accounts for the ellipticity of the electron pocket. At low $T$ the number of quasiparticles is small due to the presence of the energy gap $\Delta$, and therefore the quasiparticles do not play a role in the relaxation of $m$. This circumstance allows one to derive the time-dependent equation for $m$ in contrast to the case of conventional superconductors for which the time-dependent Ginzburg--Landau equation can be derived near $T_{\text{c}}$ only in some special cases (high concentration of paramagnetic impurities. In the stationary case the derived equation is valid at arbitrary temperatures. We find a solution of the stationary equation which describes a domain wall in the magnetic structure. In the center of the domain wall the superconducting order parameter has a maximum, which means a local enhancement of superconductivity. Using the derived time-dependent equation for $m$, we investgate also the stability of a uniform commensurate SDW and obtain the values of $\{\mu_{0}, \mu_{\phi}\}$ at which the first order transition into the state with ${m = 0}$ takes place or the transition to the state with an inhomogeneous SDW occurs.

Magnetic-Field-Tuned Aharonov-Bohm Oscillations and Evidence for Non-Abelian Anyons atν=5/2

We show that the resistance of the ν = 5/2 quantum Hall state, confined to an interferometer, oscillates with the magnetic field consistent with an Ising-type non-Abelian state. In three quantum Hall interferometers of different sizes, resistance oscillations at ν = 7/3 and integer filling factors have the magnetic field period expected if the number of quasiparticles contained within the interferometer changes so as to keep the area and the total charge within the interferometer constant. Under these conditions, an Abelian state such as the (3, 3, 1) state would show oscillations with the same period as at an integer quantum Hall state. However, in an Ising-type non-Abelian state there would be a rapid oscillation associated with the "even-odd effect" and a slower one associated with the accumulated Abelian phase due to both the Aharonov-Bohm effect and the Abelian part of the quasiparticle braiding statistics. Our measurements at ν = 5/2 are consistent with the latter.

Quasi-particle and collective magnetism: Rotation, pairing and blocking in high-K isomers

For the first time, a wide range of collective magnetic g-factors gR, obtained from a novel analysis of experimental data for multi-quasi-particle configurations in high-K isomers, is shown to exhibit a striking systematic variation with the relative number of proton and neutron quasi-particles, Np−Nn. Using the principle of additivity, the quasi-particle contribution to magnetism in high-K isomers of Lu–Re, Z=71–75, has been estimated. Based on these estimates, band-structure branching ratio data are used to explore the behavior of the collective contribution as the number and proton/neutron nature (Np, Nn), of the quasi-particle excitations, change. Basic ideas of pairing, its quenching by quasi-particle excitation and the consequent changes to moment of inertia and collective magnetism are discussed. Existing model calculations do not reproduce the observed gR variation adequately. The paired superfluid system of nucleons in these nuclei, and their excitations, present properties of general physics interest. The new-found systematic behavior of gR in multi-quasi-particle excitations of this unique system, showing variation from close to zero for multi-neutron states to above 0.5 for multi-proton states, opens a fresh window on these effects and raises the important question of just which nucleons contribute to the ‘collective’ properties of these nuclei.

Discrete-time quantum walk with nitrogen-vacancy centers in diamond coupled to a superconducting flux qubit

We propose a quantum-electrodynamics scheme for implementing the discrete-time, coined quantum walk with the walker corresponding to the phase degree of freedom for a quasi-magnon field realized in an ensemble of nitrogen-vacancy centres in diamond. The coin is realized as a superconducting flux qubit. Our scheme improves on an existing proposal for implementing quantum walks in cavity quantum electrodynamics by removing the cumbersome requirement of varying drive-pulse durations according to mean quasiparticle number. Our improvement is relevant to all indirect-coin-flip cavity quantum-electrodynamics realizations of quantum walks. Our numerical analysis shows that this scheme can realize a discrete quantum walk under realistic conditions.

Contribution of Quasiparticles in the Subgap States to the Surface Impedance of Superconductors

The surface impedance of a superconductor has been calculated using the extended Mattis-Bardeen theory, in which the complex gap energy are taken into account. It is found that the surface resistance of the superconductor increases with increasing magnitude of the imaginary part of the gap energy. It is also found that the surface resistance of an NbN film calculated by the extended Mattis-Bardeen equation quantitatively agrees well with the measured one. It is shown that temperature dependence of not only values but also residual numbers of quasiparticles of superconducting resonators agree well with those calculated by the extended Mattis-Bardeen equation. It is also shown that phonon-assisted gradual relaxation of excess quasiparticles passing through the intragap states is dominant rather than the direct recombination into Cooper pairs.

Current-Biased Kinetic Inductance Detector Using $ \hbox{MgB}_{2}$ Nanowires for Detecting Neutrons

We propose a current-biased kinetic inductance detector (CB-KID). This detector senses a change in kinetic inductance under a dc bias current, in contrast to a current-biased transition edge detector, reported previously, which probes the transient change in the bias current at the transition edge. Our CB-KID consists of a 200-nm-thick thin-film meander line with 3- wire. It is operated at 4 K. A scanning laser spot can be achieved by an XYZ piezo-driven stage and an optical fiber with an aspheric focused lens. We succeeded in observing a signal image in the contour at 4 K. The magnitude of the signal should be proportional to the number of quasi-particles excited by the laser. Because our CB-KID has a typical signal of 3-ns pulse width and can operate at 4 K, it will be possible to assemble a large-scale array of neutron detectors with single flux quantum readout circuits.

Lifshitz Transition in Kondo Alloys

We study the low-energy states of Kondo alloys as a function of the magnetic impurity concentration per site x and the conduction electron average site occupation n(c). Using two complementary approaches, the mean-field coherent potential approximation and the strong-coupling limit, we identify and characterize two different Fermi-liquid regimes. We propose that both regimes are separated by a Lifshitz transition at x=n(c). Indeed, we predict a discontinuity of the number of quasiparticles that are enclosed in the Fermi surface. This feature could provide a scenario for the non-Fermi liquid properties that were recently observed in Kondo alloy systems around x=n(c).

Decay pattern of the pygmy dipole resonance in60Ni

Spin-1 states in ${}^{60}$Ni were excited with the $(\stackrel{P\vec}{\ensuremath{\gamma}},{\ensuremath{\gamma}}^{\ensuremath{'}})$ reaction, exploiting the High Intensity $\stackrel{P\vec}{\ensuremath{\gamma}}$-ray Source at Triangle University Nuclear Laboratory. This facility is capable of providing fully linearly polarized, quasimonochromatic, Compton-backscattered photons in the entrance channel of the reaction. The depopulation of low-lying levels in an energy region far below the incident quasimonochromatic photons allows us to obtain average branching ratios of the excited spin-1 states. Levels within the energy region associated with the PDR showed regular behavior and $\ensuremath{\approx}$75$%$ of their decays are direct ground-state decays. The levels in the energy region above the PDR exhibit a statistical decay behavior to a large number of low-lying excited states and have only $\ensuremath{\approx}$50--60$%$ branches to the ground state. Within the framework of the quasiparticle phonon model this feature can be explained with the number of quasiparticles contributing to the wave functions of the excited spin-1 states. Quasimonochromatic photon beams provide a new method to test the microscopic nature of ${1}^{\ensuremath{-}}$ levels.

EFFECT OF THE STATISTICAL FLUCTUATIONS IN THE PAIRING TRANSITION IN THE CASE OF 166Er

The S-shaped heat capacity, which has recently been claimed to be a fingerprint of the superfluid-to-normal phase transition in nuclei, is studied in the case of 166 Er element. The calculations are performed by means of the modified Bardeen–Cooper–Schrieffer (MBCS) approach that takes into account the thermal fluctuation. The obtained results are compared to the experimental data and to the FT-BCS theoretical predictions. The present study illustrates the effect of the fluctuations of the quasiparticle (QP) number on the heat capacity.

Detecting Non-Abelian Anyons by Charging Spectroscopy

Observation of non-Abelian statistics for the e/4 quasiparticles in the ν = 5/2 fractional quantum Hall state remains an outstanding experimental problem. The non-Abelian statistics are linked to the presence of additional low energy states in a system with localized quasiparticles, and, hence, an additional low temperature entropy. Recent experiments, which detect changes in the number of quasiparticles trapped in a local potential well as a function of an applied gate voltage, V(G), provide a possibility for measuring this entropy, if carried out over a suitable range of temperatures, T. We present a microscopic model for quasiparticles in a potential well and study the effects of non-Abelian statistics on the charge stability diagram in the V(G)-T plane, including broadening at finite temperature. We predict a measurable slope for the first quasiparticle charging line and an even-odd effect in the diagram, which is a signature of non-Abelian statistics.

Resonating valence bonds and Fermi surface reconstruction: Resistivity in the underdoped cuprates

The pseudogap phase of the underdoped cuprates is the host to a variety of novel electronic phenomenon. An example is the dc resistivity that shows metallic behavior in the $ab$ plane, while the $c$-axis response is insulating. We apply a model, originally formulated by Yang, Rice, and Zhang [Phys. Rev. B 73, 174501 (2006)], to study the resistivity in the pseudogap phase. This model is able to reproduce the qualitative features of the resistivity, including the systematic deviations from linear behavior for the in-plane conductivity, and the insulating behavior along the $c$ axis. We compare this to the predictions of the arc model and find similar qualitative behavior. We find that the most important element in understanding the resistivity is the reconstruction of the Fermi surface, which puts strong restrictions on the number of quasiparticles allowed to participate in dc transport.

Non-equilibrium superconductivity in quantum-sensing superconducting resonators

Low temperature microwave superconducting resonators (SRs) are attractive candidates for producing quantum-sensitive, arrayable energy or power detectors for astrophysical and other precision measurement applications. Their readout uses a microwave probe signal with quanta of energy well below the threshold for pair-breaking in the superconductor. We have calculated the non-equilibrium quasiparticle and phonon distributions generated by the photons of the probe signal of a resonator operating well below its superconducting transition temperature Tc as the absorbed probe power was changed using the coupled kinetic equations described by Chang and Scalapino. The calculations give insight into a rate equation estimate which suggests that the quasiparticle distributions can be driven far from their thermal equilibrium value for typical readout powers. From the driven quasiparticle distribution functions, the driven quasiparticle number densities and lifetimes were calculated. An effective temperature to describe the driven quasiparticles was defined. The non-equilibrium lifetimes were compared to the distribution-averaged thermal lifetimes at the effective temperature and good agreement was found typically within a few per cent. We used the non-equilibrium quasiparticle distribution to model a representative SR. The complex conductivity and hence the frequency dependence of the experimentally measured forward scattering parameter S21 of the SR as a function of absorbed power were found. The non-equilibrium S21 cannot be accurately modeled by a thermal distribution even at its own elevated temperature, having a higher quality factor in all cases studied, although for low absorbed powers the two effective temperatures are similar. From the non-equilibrium lifetimes and number densities we determined the achievable noise equivalent power (NEP) of the resonator used as a power detector as a function of absorbed microwave power. Simpler expressions to evaluate the effective quasiparticle temperature as a function of absorbed power have also been derived. We conclude that multiple photon absorption from the microwave probe increases the quasiparticle number above the thermal background and ultimately limits the achievable NEP of the resonator at temperatures well below Tc.

Photons and magnetic fields: The use of synchrotron sources to study pairbreaking in superconductors

Pair-breaking effects in metallic superconductors have been studied by both linear and nonlinear spectroscopy using infrared synchrotron radiation. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory, in magnetic fields up to 10 T. The optical conductivity of thin-film superconductors in applied magnetic fields has been estimated from the results of far-infrared transmission and reflection measurements. The combined measurements have been analyzed to give the real and imaginary parts of the conductivity. In turn, these quantities allow the magnetic-field dependence of the superconducting energy gap, pairbreaking parameter, and superfluid density to be estimated. Photons also may break Cooper pairs. Pump-probe studies of excess quasiparticle relaxation in the these superconducting films show a relaxation rate proportional to the excess quasiparticle number density, as expected for bimolecular recombination driven by a large excess quasiparticle population. Application of a magnetic field parallel to the sample surface is found to slow significantly the quasiparticle recombination process.