Pj Meeson

Decoherence in a superconducting quantum bit circuit

Decoherence in quantum bit circuits is presently a major limitation to their use for quantum computing purposes. We present experiments, inspired from NMR, that characterize decoherence in a particular superconducting quantum bit circuit, the quantronium. We introduce a general framework for the analysis of decoherence, based on the spectral densities of the noise sources coupled to the qubit. Analysis of our measurements within this framework indicates a simple model for the noise sources acting on the qubit. We discuss various methods to fight decoherence.

Quantum oscillations in the mixed state of the type II superconductor 2H-NbSe2

Quantum oscillations have been investigated far into the mixed state of the type II superconductor 2H-NbSe2. The de Haas-van Alphen effect is found to persist to fields as low as approximately 0.3BC2 and to temperatures of approximately 0.003TC. A self-consistent band structure calculation, performed for 2H-NbSe2 in the normal non-charge-density-wave state using a full potential linearized augmented plane wave (FLAPW) method, accounts well for the experimental angular dependence of the quantum oscillations in terms of a small flat hole Fermi surface around the Gamma point. This feature derives mainly from chalcogen p bands and has a mass-enhancement factor much smaller than expected from the specific heat. Quantum oscillations in the presence of the vortex lattice are observed to experience an additional damping, from which the order parameter is estimated to be Delta (0)=0.6+or-0.1 meV.

de Haas – van Alphen effect in single crystal MgB 2

Abstract We present an experimental study of the de Haas–van Alphen effect in single crystals of MgB 2 , using a piezo-resistive torque technique. Three quasi-particle orbits were observed. Two originate from a single warped Fermi surface tube along the c direction, and the third from a cylindrical section of an in-plane honeycomb network. The quasi-particle effective masses on these orbits were determined and compared to band structure calculations. From this we deduce that the electron–phonon coupling strength λ , is a factor ∼3 larger for the c -axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.

Evidence for interacting two-level systems from the 1/f noise of a superconducting resonator

The performance of a great variety of electronic devices--ranging from semiconductor transistors to superconducting qubits--is hampered by low-frequency noise with spectra proportional to 1/f. The ubiquity and negative impact of 1/f noise has motivated intensive research into its cause, and it is now believed to originate from a bath of fluctuating two-level defect states (TLSs) embedded in the material. This phenomenon is commonly described by the long-established standard tunnelling model (STM) of independent TLS. A key prediction of STM is that the noise should vanish at low temperatures. Here we report measurements on superconducting microresonators over previously unattainable, very long time scales that show an increase in 1/f noise at low temperatures and low microwave power, contrary to the STM. We propose a new generalised tunnelling model that includes significant interaction between multiple TLSs, which fully describes these observations, as well as recent studies of individual TLS lifetimes in superconducting qubits.

Determination of the Fermi surface of MgB2 by the de Haas-van Alphen effect.

We report measurements of the de Haas-van Alphen (dHvA) effect for single crystals of MgB2, in magnetic fields up to 32 T. In contrast to our earlier work, dHvA orbits from all four sheets of the Fermi surface were detected. Our results are in good overall agreement with calculations of the electronic structure and the electron-phonon mass enhancements of the various orbits, but there are some small quantitative discrepancies. In particular, systematic differences in the relative volumes of the Fermi-surface sheets and the magnitudes of the electron-phonon coupling constants could be large enough to affect detailed calculations of T(c) and other superconducting properties.

Heavy quasiparticles in CeCu6 studied using magnetic quantum oscillations

Angle resolved measurements of the de Haas-van Alphen effect are reported in the heavy-fermion compound CeCu6. The experiments confirm the presence at low temperature of a sharply defined Fermi surface resulting from the existence of heavy long-lived charged fermion quasiparticles. LMTO electronic structure calculations have been performed within the local density approximation, in which the f electrons are included as either band or core states, but in neither case are the results in satisfactory agreement with experiment. The measured effective masses, which are in the range 6-80 me, are found to be magnetic field dependent in a manner which closely resembles that of the linear specific heat coefficient. The authors deduce that, in zero applied field, the mass renormalization in CeCu6 is greater than 200.

De Haas-van Alphen effect in the superconducting state

Abstract Quantum oscillations were first observed in the mixed state of superconductor, NbSe 2 , by Graebner and Robbins in 1976. More recently, the investigation of a number of other materials has shown that de Haas-van Alphen effect oscillations persist below B c2 in many systems, so that quantum oscillations in the vortex state appear to be the rule rather than the exception. Such measurements demonstrate that the effects of Landau quantization persist in the mixed state of a clean type II superconductor. In this paper we review work on NbSe 2 , V 3 Si and Nb 3 Sn, for which detailed measurements exist. We find that the principal effect of the superconducting order parameter is to reduce the amplitude of the oscillatory magnetisation. In principle, quantum oscilltory measurements of this type will give detailed information on the anisotropies of the superconducting state.

Heavy quasiparticles in the ferromagnetic superconductor ZrZn2

We report a study of the de Haas-van Alphen effect in the normal state of the ferromagnetic superconductor ZrZn2. Our results are generally consistent with a linear muffin-tin orbital band structure which predicts four exchange-split Fermi surface sheets. Quasiparticle effective masses are enhanced by a factor of 4.9 implying a strong coupling to magnetic excitations or phonons. ZrZn2 is unique among metallic ferromagnets in that it has a very large density of states in the ferromagnetic phase.

Quasiparticle and thermodynamic mass in the heavy-fermion system CeB6

The de Haas-van Alphen effect has been investigated in the heavy-fermion compound CeB6 in magnetic fields extending to 53 T. Absolute amplitude measurements reveal that the major sheets of the Fermi surface are not spin polarized, and that its observed volume is 1.15+or-0.1 electrons per formula unit. Analysis of the field dependence of the quasiparticle effective mass based on the periodic Anderson model yields, for the ratio of the characteristic temperature to the g factor, T*/g = 4.0 +or- 0.8 K, while over the field interval 31-43 T, for which the mass changes by a factor of two, the mean free path of the quasiparticles remains constant at 56 +or- 1 nm. We compare the density of states derived from the measured quasiparticle masses with that deduced from specific-heat experiments for magnetic fields below approximately 30 T, and find them to be in agreement. We conclude that the low-temperature thermodynamics of CeB6 may be understood simply in terms of the low-energy excitations of a heavy charged Fermi liquid. We note that these experiments do not support the idea of Kagan et al that a significant contribution to the low-temperature thermodynamics of Ce-based heavy-fermion systems arises from the presence in them of a neutral Fermi liquid.

de Haas-van Alphen effect in MgB2 crystals

Abstract In this paper we review and expand our previous report concerning the observation of de Haas–van Alphen (dHvA) oscillations in single crystals of the high temperature superconductor MgB 2 . The analysis of these oscillations provides an important and precise check of band-structure calculations, as well as a measure of the many body electron–phonon interactions, which enhance the quasi-particle effective masses and give rise to superconductivity. Orbits are observed on both the σ and π bands, with dHvA frequencies in good agreement with recent calculations. By comparing the measured quasi-particle effective masses with band masses calculated recently by three groups, we find that the electron–phonon coupling strength λ is a factor ∼3 larger on the 2D σ orbits than on the 3D π orbits, in accord with recent microscopic calculations. We are therefore able to verify two of the key ingredients in the two-gap model of superconductivity in this compound, namely the Fermi surface topology and the disparity in the electron–phonon coupling for the σ and π bands.