Cooper Pair Breaking Research Articles

Properties of warm nuclei in the quasi-continuum

Nuclear thermodynamic quantities are extracted from nuclear level densities measured with the CACTUS detector array at the Oslo Cyclotron Laboratory. The experiments are performed with light-particle inelastic or transfer reactions. A simple combinatorial model is used to describe the underlying mechanisms responsible for the exponential increasing level density as function of excitation energy. The calculated number of broken Cooper pairs and the parity distribution in continuum are discussed.

Terahertz detector based on a superconducting tunnel junction coupled to a thin superconductor film

The principle of a terahertz detector using a superconducting tunnel junction (STJ) coupled to a large terahertz-absorbing superconductor film is verified. We have detected terahertz radiation based on the Cooper-pair breaking process, and confirmed that the sensitivity has a sharp increase around 0.7 THz, a value that is in agreement with the gap frequency of the superconducting Nb. For high-sensitivity terahertz detection, we propose an improved design of the STJ detector composed of a smaller junction coupled to a thinner superconductor film. We also discuss the expected noise equivalent power of the optimized detector.

Unambiguous probe of parity mixing of Cooper pairs in noncentrosymmetric superconductors

We propose an experimental scheme to detect unambiguously parity-mxing of Cooper pairs in noncentrosymmetric superconductors, which utilizes crossed Andreev reflection processes between two oppositely spin-polarized normal metal leads and a noncentrosymmetric superconductor. It is demonstrated that a non-local conductance exhibits a clear signature of parity breaking of Cooper pairs, and thus, can be a direct probe for the parity-mixing.

Rotation-induced superfluid-normal phase separation in trapped Fermi gases

We use the Bogoliubov-de Gennes formalism to analyze the effects of rotation on the ground state phases of harmonically trapped Fermi gases, under the assumption that quantized vortices are not excited. We find that the rotation breaks Cooper pairs that are located near the trap edge, and that this leads to a phase separation between the nonrotating superfluid (fully paired) atoms located around the trap center and the rigidly rotating normal (nonpaired) atoms located towards the trap edge, with a coexistence (partially paired) region in between. Furthermore, we show that the superfluid phase that occurs in the coexistence region is characterized by a gapless excitation spectrum, and that it is distinct from the gapped phase that occurs near the trap center.

Current-induced instability of superfluid Fermi gases in optical lattices

We study the stability of superfluid flow of two-component Fermi gases in one-dimensional optical lattices. We find that the density fluctuation mode, the so-called Anderson-Bogoliubov mode, has the roton-like structure as seen in superfluid 4He. With increasing supercurrent, one of the roton-like minima reaches zero before the pair breaking occurs. This means that the instability of the superfluid Fermi gas is due to the spontaneous emission of the roton-like excitations of the Anderson-Bogoliubov mode instead of due to the Cooper pair breaking. We calculate the critical velocity determined by the roton-like structure for one-dimensional optical lattices.

Thermodynamic properties ofFe56,57

Nuclear level densities for $^{56,57}\mathrm{Fe}$ have been extracted from the primary $\ensuremath{\gamma}$-ray spectra using (${}^{3}\mathrm{He}, {}^{3}{\mathrm{He}}^{'}\ensuremath{\gamma}$) and (${}^{3}\mathrm{He}, \ensuremath{\alpha}\ensuremath{\gamma}$) reactions. Nuclear thermodynamic properties for $^{56}\mathrm{Fe}$ and $^{57}\mathrm{Fe}$ are investigated using the experimental level densities. These properties include entropy, Helmholtz free energy, caloric curves, chemical potential, and heat capacity. In particular, the breaking of Cooper pairs and single-quasiparticle entropy are discussed and shown to be important concepts for describing nuclear level density. Microscopic model calculations are performed for level densities of $^{56,57}\mathrm{Fe}$. The experimental and calculated level densities are compared. The average number of broken Cooper pairs and the parity distribution are extracted as a function of excitation energy for $^{56,57}\mathrm{Fe}$ from the model calculations.

Theory of the pairbreaking superconductor–metal transition in nanowires

We present a detailed description of a zero temperature phase transition between superconducting and diffusive metallic states in very thin wires due to a Cooper pair breaking mechanism. The dissipative critical theory contains current reducing fluctuations in the guise of both quantum and thermally activated phase slips. A full cross-over phase diagram is computed via an expansion in the inverse number of complex components of the superconducting order parameter (one in the physical case). The fluctuation corrections to the electrical ( σ) and thermal ( κ) conductivities are determined, and we find that σ has a non-monotonic temperature dependence in the metallic phase which may be consistent with recent experimental results on ultra-narrow wires. In the quantum critical regime, the ratio of the thermal to electrical conductivity displays a linear temperature dependence and thus the Wiedemann–Franz law is obeyed, with a new universal experimentally verifiable Lorenz number.

Neutrino emission due to Cooper-pair recombination in neutron stars reexamined

Neutrino emission in processes of breaking and formation of neutron and proton Cooper pairs is calculated within the Larkin-Migdal-Leggett approach for a superfluid Fermi liquid. We demonstrate explicitly that the Fermi-liquid renormalization respects the Ward identity and ensures the weak vector-current conservation. The systematic expansion of the emissivities for small temperatures and nucleon Fermi velocity ${v}_{\mathrm{F},i},i=n,p,$ is performed. Both neutron and proton processes are mainly controlled by the axial-vector current contributions, which are not strongly changed in the superfluid matter. Thus, compared to earlier calculations, the total emissivity of processes on neutrons paired in the $1{S}_{0}$ state is suppressed by a factor of $\ensuremath{\simeq}(0.9\text{\ensuremath{-}}1.2){v}_{\mathrm{F},n}^{2}$. A similar suppression factor ($~{v}_{\mathrm{F},p}^{2}$) arises for processes on protons.

Superconducting detectors for particles from atoms to proteins

Superconducting detectors are ideal for detecting low-energy particles from atoms to proteins of which kinetic energies range from less than 1 keV to ∼30 keV compatible with analytical instruments for ordinary laboratory use. The superconducting detectors have the advantages of detecting low energy quanta, because a threshold equivalent to the gap parameter (Δ) is extremely small: ∼meV that is one-thousandth of the band gap of semiconductor detectors. Any low-energy quanta including phonons created by individual particle hit-events on the detector surface can be detected through Cooper-pair breaking. Such detectors as superconducting tunnel junctions (STJs) and superconducting stripline detectors (SSLDs) are promising for developing advanced analytical instruments for nuclear physics, basic chemistry, and biology. Especially, mass spectrometry (MS) equipped with the superconducting detectors has been producing the remarkable results, which have never been obtained by conventional MS instruments.

Quasiparticle relaxation dynamics in n-type superconductor [formula omitted

We have studied the photoexcited carrier dynamics in the electron-doped La 2 - x Ce x CuO 4 (LCCO). In general, there is a slowing down of the relaxation as the temperature decreases. However, the quasiparticle lifetime is found to exhibit a quasi-divergence as the temperature approaches T c from below. We have also observed picosecond rise time in the electron-doped LCCO, which is attributed to the Cooper pair breaking dynamics. The experimental results are analyzed by Rothwarf–Taylor model.

Observation of Nonmagnetic Resonant Scattering Effects by Tunneling in Dilute Al-Mn Alloy Superconductors

We have observed the BCS-like density of states predicted for energy-gap suppression by nonmagnetic Anderson impurities in superconductors. We show that Mn impurities in Al exhibit no magnetic character and act exclusively as strong resonant scattering sites without producing time-reverse symmetry breaking of Cooper pairs (pair breaking).

Vertex renormalization of weak interactions and Cooper-pair breaking in cooling compact stars

Below the critical temperature of superfluid phase transition baryonic matter emits neutrinos by breaking and recombination of Cooper pairs formed in the condensate. The weak vector and axial-vector vertices and the neutrino loss rates via pair breaking are modified by strong interactions in nuclear medium. We study these modifications nonperturbatively by summing infinite series of particle-hole loops in $S$-wave superfluid neutron matter. The interactions in the particle-hole channel are described within the Landau Fermi-liquid framework with the Landau parameters derived from the microscopic theory. The $S$-wave superfluid is described within the BCS theory. We derive the renormalized three-point vector and axial-vector vertices and the complete polarization tensor of matter and its low momentum transfer expansion. The leading-order term in this expansion and the associated neutrino losses arise at $O({q}^{2})$, consistent with the $f$-sum rule. The neutrino emission rate due to the pair breaking is parametrically suppressed compared to its one-loop counterpart by the ratio of the neutron recoil energy to the temperature, which is of order $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. The approximations to the normal and anomalous self-energies that guarantee the conformity of the theory with the generalized Ward identities are established.

Microscopic Study of Nuclear Level Densities and ThermalProperties in 56,57Fe

We calculate level densities for 56,57Fe nuclei using BCShamiltonian with inclusion of pairing interaction. The results ofcalculations show that the step structure observed experimentallyhas been supported by microscopic theory. Also the S-shaped energyand entropy as function of temperature have been obtained intheoretical calculations. Structures in the curves are interpretedas fingerprints of breaking Cooper pairs and quenching of pairing correlations.

Incoherent non-Fermi-liquid scattering in a Kondo lattice

One of the most notorious non-Fermi-liquid properties of both archetypal heavy-fermion systems1,2,3,4 and the high-Tc copper oxide superconductors5 is an electrical resistivity that evolves linearly (rather than quadratically) with temperature, T. In the heavy-fermion superconductor CeCoIn5 (ref. 6), this linear behaviour was one of the first indications of the presence of a zero-temperature instability, or quantum critical point. Here, we report the observation of a unique control parameter of T-linear scattering in CeCoIn5, found through systematic chemical substitutions of both magnetic and non-magnetic rare-earth, R, ions into the Ce sublattice. We find that the evolution of inelastic scattering in Ce1−xRxCoIn5 is strongly dependent on the f-electron configuration of the R ion, whereas two other key properties—Cooper-pair breaking and Kondo-lattice coherence—are not. Thus, T-linear resistivity in CeCoIn5 is intimately related to the nature of incoherent scattering centres in the Kondo lattice, which provides insight into the anomalous scattering rate synonymous with quantum criticality7.

BaO PLANES, NOT CuO2 PLANES, CONTAIN HIGH-TC SUPERCONDUCTIVITY

Muon spin rotation (μ+SR) measurements conducted on crystalline YBa 2 Cu 3 O 7 are consistent with s-wave pairing, not d-wave, suggesting that the superconducting hole condensate resides in the BaO layers, not in the cuprate-planes. The specific heat and thermal conductivity data are explained by the superconducting BaO layers alone, unlike the failed interpretation based on CuO 2-plane superconductivity. The layer charges of the CuO 2 planes are almost -2 | e |, indicating that those planes are primarily carriers of electrons, not holes. The cuprate-planes are not the dominant hole-carriers of high- T C superconductivity, as demonstrated by doped YBa 2 RuO 6, which has no such CuO 2 lanes, yet superconducts at ~ 93 K. Moreover the trio of related compounds, YSr 2 RuO 6 (doped with Cu on Ru sites), undoped GdSr 2 Cu 2 RuO 8, and undoped Gd 2-z Ce z Sr 2 Cu 2 RuO 10 all start superconducting near 49 K in their SrO layers, not in the cuprate planes of the two compounds that have such planes, because those planes are either antiferromagnetic or weakly ferromagnetic and so do not superconduct. In PrBa 2 Cu 3 O 7, a Pr -on- Ba -site ( Pr Ba) defect kills the superconductivity, but Pr -on- Pr -site ( Pr Pr) does not. Both defects are approximately equidistant from the intervening cuprate plane, suggesting that the cuprate plane does not carry significant superconductivity. In GdBa 2 Cu 3 O 7, Gd -on-a- Gd -site ( Gd Gd) does not break Cooper pairs, but Gd -on-a- Ba -site ( Gd Ba) does, indicating that the superconductivity is in the BaO layers, and not in the cuprate-planes. In HgBa 2 Ca n-1 Cu n O 2n+2, the BaO layers, not the cuprate-planes, gain positive charge as T C, pressure, and the number of layers n increase. The reason that theories based on holes in the cuprate-planes have done so poorly is that those planes were incorrectly identified as the source of high-temperature superconductivity on the basis of a single datum by Cava et al., that was first contradicted by Jorgensen et al., and then endorsed by Jorgensen alone on the basis of the Cava datum, not his own. This error is the main reason why cuprate-plane superconductivity, which probably does not exist, has been so widely accepted.

Hot-Phonon Controlled MicroSQUIDs With Independently Controlled Junctions

The microSQUID is a particular type of DC-SQUID notable both for its small loop area and the use of microbridges as Josephson junctions. Hot phonons can be used to reduce the critical current in a superconducting microbridge and hence allow electrical control of the SQUID parameters. A normal metal layer, isolated from the superconductor by a thin dielectric film, generates non-equilibrium hot phonons that travel a short distance before thermalisation. These hot phonons are very effective in breaking Cooper pairs, thereby suppressing the order parameter and reducing the junction critical current. Phonon control of a microSQUID has previously been demonstrated in which both junctions were influenced by the same control current, allowing control of the magnetic hysteresis. Here we report a modified fabrication process, allowing independent control of the microbridge junctions. The fabrication of the devices involves several stages of electron beam lithography, deposition and reactive ion etching. The superconductor used is a sputtered niobium film, isolated from the normal metal, phonon-producing layer by a dielectric SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film. Separate lithography stages form the normal metal wires and the rest of the SQUID loop, which then act as an etch mask for a CF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> and O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reactive ion etch used to define the SQUID in the niobium layer.

Mechanism of Light Detection in Current-Biased Superconducting ${\rm MgB}_{2}$ Microbridges

We report on time-resolved, transient photoimpedance measurements performed on current-biased microbridges excited by 100-fs wide, 800-nm wavelength, and 76-MHz repetition-rate optical pulses. Our bridges had dimensions of 20 mum by 40 mum and were patterned by conventional photolithography and ion milling from 150-nm-thick, epitaxial MgB2 films grown by a hybrid physical-chemical vapor-deposition technique. For high-speed detection experiments, the microbridges were embedded into coplanar-strip transmission lines and connected to a 20-GHz-bandwidth sampling oscilloscope via a semirigid coaxial cable. The test structures were placed inside an optical cryostat and studies were performed in the temperature range between 15 K and 40 K. At temperatures, e.g., 20 K, far below the bridge critical temperature and under low optical excitation, large and fast (130 ps in duration) photoresponse transients were observed, which we associate with the kinetic-inductive (Cooper-pair breaking) mechanism. At temperatures approaching the MgB2 critical temperature and/or under intense light illumination, an additional slow component of the photoresponse with a nanosecond decay time was measured. We interpret this second photoimpedance component as a resistive response, resulting from the hot-electron heating effect. The observed picosecond kinetic photoresponse makes our MgB2 bridges attractive candidates for fast and efficient optical detectors and photon counters.

Critical temperature and specific heat for Cooper pairing on a spherical surface

Based on an exact solution of the Bardeen-Cooper-Schrieffer type Hamiltonian on a spherical surface, we calculate the specific heat for the electron system with pair correlations on a sphere. We find that it is possible to extract from the specific heat a temperature above which many-body states with broken Cooper pairs get populated. Therefore, we define this temperature as the characteristic temperature signalling the onset of a BCS-type pair-correlated state for electrons on a spherical surface. Such spherical electron systems are realized in multielectron bubbles in liquid helium, for which the above-mentioned characteristic temperature is found to be of the order of 10-100 mK. Both the specific heat and the critical temperature show a pronounced (4-6%) odd-even parity effect that persists even for numbers of electrons as large as 10$^6$.

Thermal evolution of a rotating strange star in the colour superconductivity phase

Under the combination effect of recommencement heating due to the spin-down of strange stars (SSs) and heat preservation due to the weak conduction heat of the crust, Cooper pair breaking and formation (PBF) in colour superconducting quark matter arises. We investigate the cooling of SSs with a crust in the colour superconductivity phase including both deconfinement heating (DH) and the PBF process. We find that DH can delay the thermal evolution of SSs and the PBF process suppresses the early temperature rise of the stars. The cooling SSs behave within the brightness constraint of young compact objects when the colour superconductivity gap is small enough.

Nature of high-temperature superconductivity

Evidence is presented that the superconducting hole condensate generally does not reside in the cuprate planes of high-temperature superconductors, but in the SrO layers, in the BaO layers, or in the regions of interstitial oxygen. Evidence that electrons, not holes, transfer to the cuprate planes of HgBa2Can−1CunO2+n+δ as a function of pressure, number n of layers, and increasing Tc is presented; holes transfer to the BaO layers. The hole transfer in YBa2Cu3O7 is also to the BaO layers. PrBa2Cu3O7 superconducts (as predicted) when it is free of pair-breaking PrBa defects in its BaO layers. The chosen locus of the superconductivity is consistent with the observation of magnetism in both the CuO layers and the cuprate planes of YBa2Cu3O7. Four materials were successfully predicted to superconduct by assuming that the cuprate planes are normal. There are no n-type high-temperature superconductors; Nd2−zCezCuO4 is p type and doped with interstitial oxygen. When Y+3 is replaced by Am+4, Pb2Sr2YCu3O8 becomes n type and stops superconducting. Holes remain near interstitial oxygen in Tl2Ba2Can−1CunO2n+4+δ. Gd2−zCezCuO4, unlike Nd2−zCezCuO4, does not superconduct because Gd has L=0 and J≠0 and breaks Cooper pairs associated with its interstitial oxygen, but Gd2−zCezSr2Cu2NbO10 does superconduct (in its SrO layers). YBa2Cu3O7 exhibits bulk nodeless (s-wave) superconductivity. We argue that the superconductivity of YBa2Cu3O7 is representative of high-Tc superconductors. The pairing mechanism is electronic (not phononic) and associated with holes on certain oxygen ions (or sulfur ions, in the case of some organic superconductors). We explore a Bardeen-Cooper-Schrieffer-type formalism applied to cuprates, ruthenates, and other compounds.