Cooper Pair Breaking Research Articles

Development of Microwave Kinetic Inductance Detectors for a Detection of Phonons

We present a status of the development of microwave kinetic inductance detectors (MKIDs) for a detection of athermal phonons in a substrate. The energy deposited in the substrate is converted to athermal phonons. Athermal phonons arriving at the surface can break Cooper pairs in the MKIDs which are formed as a thin superconducting metal layer in the substrate surface. By counting the number of Cooper pairs broken and measuring the phonon arrival times, we can measure the amount of deposited energy and its position. MKIDs are suitable for the frequency-domain multiplexing readout, which enables us to readout hundreds of pixels simultaneously and, hence, to detect athermal phonons with a large detection efficiency. We fabricated MKIDs with a combination of aluminum and niobium on a silicon substrate, and then irradiated it with \(\alpha \) particles from an \(^{241}\)Am source. We detected phonons and made a rough estimation of the phonon propagation velocity of 1.1–1.3 km/s. We found that a thin insulator layer can block the phonon propagation from the substrate to the thin metal layer.

THE COOLING OF THE CASSIOPEIA A NEUTRON STAR AS A PROBE OF THE NUCLEAR SYMMETRY ENERGY AND NUCLEAR PASTA

X-ray observations of the neutron star in the Cas A supernova remnant over the past decade suggest the star is undergoing a rapid drop in surface temperature of $\approx$ $2-5.5\%$. One explanation suggests the rapid cooling is triggered by the onset of neutron superfluidity in the core of the star, causing enhanced neutrino emission from neutron Cooper pair breaking and formation (PBF). Using consistent neutron star crust and core equations of state (EOSs) and compositions, we explore the sensitivity of this interpretation to the density dependence of the symmetry energy $L$ of the EOS used, and to the presence of enhanced neutrino cooling in the bubble phases of crustal "nuclear pasta". Modeling cooling over a conservative range of neutron star masses and envelope compositions, we find $L\lesssim70$ MeV, competitive with terrestrial experimental constraints and other astrophysical observations. For masses near the most likely mass of $M\gtrsim 1.65 M_{\odot}$, the constraint becomes more restrictive $35\lesssim L\lesssim 55$ MeV. The inclusion of the bubble cooling processes decreases the cooling rate of the star during the PBF phase, matching the observed rate only when $L\lesssim45$ MeV, taking all masses into consideration, corresponding to neutron star radii $\lesssim 11$km.

In-gap quasiparticle excitations induced by non-magnetic Cu impurities in Na(Fe(0.96) Co(0.03)Cu(0.01))As revealed by scanning tunnelling spectroscopy.

The origin of superconductivity in the iron pnictides remains unclear. One suggestion is that superconductivity in these materials has a magnetic origin, which would imply a sign-reversal s± pairing symmetry. Another suggests it is the result of orbital fluctuations, which would imply a sign-equal s++ pairing symmetry. There is no consensus yet which of these two distinct and contrasting pairing symmetries is the right one in iron pnictide superconductors. Here we explore the nature of the pairing symmetry in the superconducting state of Na(Fe0.97−xCo0.03Cux)As by probing the effect of scattering of Cooper pairs by non-magnetic Cu impurities. Using scanning tunnelling spectroscopy, we identify the in-gap quasiparticle states induced by the Cu impurities, showing signatures of Cooper pair breaking by these non-magnetic impurities–a process that is only consistent with s± pairing. This experiment provides strong evidence for the s± pairing.

Postadiabatic Hamiltonian for low-energy excitations in a slowly-time-dependent BCS-BEC crossover

We develop a Hamiltonian that describes the time-dependent formation of a molecular Bose-Einstein condensate from a Bardeen-Cooper-Schrieffer state of fermionic atoms as a result of slowly sweeping through a Feshbach resonance. In contrast to many other calculations in the field, our Hamiltonian includes the leading postadiabatic effects that arise because the crossover proceeds at a nonzero sweep rate. We apply a path-integral approach and a stationary phase approximation for the molecular $\mathbf{k}=\mathbf{0}$ background, which is a good approximation for narrow resonances [see, e.g., Diehl and Wetterich, Phys. Rev. A 73, 033615 (2006) as well as Diehl, Gies, Pawlowski, and Wetterich, Phys. Rev. A 76, 053627 (2007)]. We use two-body adiabatic approximations to solve the atomic evolution within this background. The dynamics of the $\mathbf{k}\ensuremath{\ne}\mathbf{0}$ molecular modes is solved within a dilute gas approximation and by mapping it onto a purely bosonic Hamiltonian. Our main result is a postadiabatic effective Hamiltonian in terms of the instantaneous bosonic (Anderson-)Bogoliubov modes, which holds throughout the whole resonance, as long as the Feshbach sweep is slow enough to avoid breaking Cooper pairs.

Non-equilibrium Beta Processes in Neutron Stars Containing Color-Superconductivity Quark Cores

The effect of quark color-superconductivity on the reactions rate and emissivity of nonequilibrium beta processes is investigated. We calculate the implied reduction factors for the direct Urca processes, with three flavors of quarks all pairing with the same gap. The numerical results show that the color-superconductivity dramatically suppress the Durca reaction rate and emissivity, and the reduction is also related to the chemical departure from beta equilibrium. For small chemical departure, the reduction factor almost only depends on the gap. Until the departure reaches a certain value, it enhances the reaction rate and emissivity rapidly. Furthermore, the chemical departure may be the cause of the breaking of quark Cooper pairs formed between different flavors of quarks. These results may be used in the cooling simulation of neutron stars, which contain a color-superconductivity quark core in the non-equilibrium state.

Soft Superconducting Gap in Semiconductor Majorana Nanowires

We theoretically consider the ubiquitous soft gap measured in the tunneling conductance of semiconductor-superconductor hybrid structures, in which recently observed signatures of elusive Majorana bound states have created much excitement. We systematically study the effects of magnetic and nonmagnetic disorder, temperature, dissipative Cooper pair breaking, and interface inhomogeneity, which could lead to a soft gap. We find that interface inhomogeneity with moderate dissipation is the only viable mechanism that is consistent with the experimental observations. Our work indicates that improving the quality of the superconductor-semiconductor interface should result in a harder induced gap.

Isotope exponent in disordered underdoped and overdoped La214

The effect of oxygen isotope substitution on Tc has been analyzed for heavily underdoped and overdoped La2-xSrxCu1-yZnyO4 compounds with different Zn contents in the CuO2 plane. The effect of Zn on the isotope exponent, α, was more pronounced in the case of the underdoped (x = 0.09) compounds compared to the overdoped (x = 0.22) ones. The variation of α with in-plane disorder can be described quite well within a model based solely on impurity induced Cooper pair-breaking in the case of the underdoped compounds. This model, on the other hand, fails to describe the behavior of α(y) for the overdoped samples, even though Zn still breaks pairs quite effectively at this hole (Sr) content. We discuss the implications of these finding in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as number of charge carriers in the CuO2 plane, p (≡ x), is varied.

Axions from cooling compact stars: Pair-breaking processes

Once formed in a supernova explosion, a neutron star cools rapidly via neutrino emission during the first 10^4-10^5 yr of its life-time. Here we compute the axion emission rate from baryonic components of a star at temperatures below their respective critical temperatures T_c for normal-superfluid phase transition. The axion production is driven by a charge neutral weak process, associated with Cooper pair breaking and recombination. The requirement that the axion cooling does not overshadow the neutrino cooling puts a lower bound on the axion decay constant f_a > 6 10^9 T_{c9}^{-1} GeV, with T_{c9} = T_c/10^9 K. This translates into a upper bound on the axion mass m_a < 10^{-3} T_{c9} eV.

Terahertz detector with transmission-line type superconducting tunnel junctions

We demonstrate a new type of terahertz detector with superconducting tunnel junctions. The detector has two long junctions integrated on both wings of a log-periodic antenna. In this type of detector, the long junction is a lossy transmission line working based on the Cooper-pair breaking, as well as a standing-wave resonance line working based on the photon-assisted tunneling. A prototype detector using Nb/Al/AlOx/Al/Nb junctions was fabricated, and the principle of the detector was verified. The prototype shows a gradual increase in sensitivity starting from 0.35 THz. We have also identified a resonance signal at 0.25 THz below the effective gap frequency.

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.

Effect of DC current injection on AC supercurrent carrying ability of ring shaped HTS thin films

The effect of DC current injection on the AC current carrying characteristics of superconducting thin film rings was analyzed because of the possibility for using similar ferromagnetic/high temperature superconducting (FM/HTS) structures for contactless investigation of the spin-injection process. It was shown, that the injection leads to decreasing of the critical current I C1, determined by using AC magnetic field. A superposition of AC and DC currents in the HTS ring and a breaking of Cooper pairs may cause this effect. The effect was investigated experimentally at 77 K in HTS Y 1Ba 2Cu 3O 7− x (YBCO) thin film samples with YBCO or La 0.7Sr 0.3MnO 3 (LSMO) current injecting electrodes: I C1 of the last sample was found to be more sensitive to the injection process. The results allow concluding, that a change of the I C1 caused by both mechanisms can be evaluated separately and the contactless method can be successfully used for the investigation of the spin-injection effect.

Photoinduced melting of superconductivity in the high-Tcsuperconductor La2−xSrxCuO4probed by time-resolved optical and terahertz techniques

Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4 thin films is investigated utilizing optical pump-probe and optical pump - THz probe techniques as a function of temperature and excitation fluence. The absorbed energy density required to suppress superconductivity is found to be about 8 times higher than the thermodynamically determined condensation energy density and nearly temperature independent between 4 and 25 K. These findings indicate that during the time when superconducting state suppression takes place (~0.7 ps), a large part (nearly 90%) of the energy is transferred to the phonons with energy lower than twice the maximum value of of the SC gap and only 10% is spent on Cooper pair breaking.

Rapid Cooling of the Neutron Star in Cassiopeia A Triggered by Neutron Superfluidity in Dense Matter

We propose that the observed cooling of the neutron star in Cassiopeia A is due to enhanced neutrino emission from the recent onset of the breaking and formation of neutron Cooper pairs in the (3)P(2) channel. We find that the critical temperature for this superfluid transition is ≃0.5×10(9) K. The observed rapidity of the cooling implies that protons were already in a superconducting state with a larger critical temperature. This is the first direct evidence that superfluidity and superconductivity occur at supranuclear densities within neutron stars. Our prediction that this cooling will continue for several decades at the present rate can be tested by continuous monitoring of this neutron star.

Oxygen isotope effect in disordered underdoped and overdoped La 2−xSr xCu 1−yZn yO 4 superconductors

The effect of oxygen isotopic substitution on the superconducting transition temperature has been studied for heavily underdoped and overdoped La 2− x Sr x Cu 1− y Zn y O 4 compounds with different Zn contents in the CuO 2 plane. The effect of Zn on the isotope coefficient, α, was significantly more pronounced in the case of the underdoped ( x = 0.09) compounds compared to the overdoped ( x = 0.22) ones. The variation of α with disorder content can be described quite well within a model based solely on Cooper pair-breaking in the case of the underdoped compounds. This model fails to describe the behavior of α( y) for the overdoped samples, even though Zn still suppresses T c very effectively at this hole (Sr) content, indicating that the Zn induced pair-breaking is still very much at play. We discuss the implications of these findings in details by considering the Zn induced magnetism, stripe correlations, and possible changes in the superconducting order parameter as hole content in the CuO 2 plane, p (≡ x), is varied.

Simulated radiation effects in the superinsulating phase of titanium nitride films

This paper investigates possible effects of alpha particle and ion beam irradiation on the properties of the superinsulating phase, recently observed in titanium nitride films, by using numerical simulation of particle transport. Unique physical properties of the superinsulating state are considered by relying on a two-dimensional Josephson junction array as a model of material structure. It is suggested that radiation-induced change of the Josephson junction charging energy would not affect the current-voltage characteristics of the superinsulating film significantly. However, it is theorized that a relapse to an insulating state with thermally activated resistance is possible, due to radiation-induced disruption of the fine-tuned granular structure. The breaking of Cooper pairs caused by incident and displaced ions may also destroy the conditions for a superinsulating phase to exist. Finally, even the energy loss to phonons can influence the superinsulating state, by increasing the effective temperature of the phonon thermostat, thereby reestablishing means for an energy exchange that can support Cooper pair tunneling.

A comparative study of Pr substitution at Y and Ba sites in YBa 2Cu 3O 7 − δ

Aimed at identifying the factors which suppress the superconductivity in cuprate perovskites I have investigated the effect of substituting Pr ions at the Y and Ba sites in YBa 2Cu 3O 7 − δ system using structural, transport, iodometric, and photoemission studies. The rate of Tc depression in the case when Pr substitutes the Ba site is much higher than the case when it substitutes the Y. This is explained as being due to a combined effect of the factors such as depletion of itinerant holes due to depletion of the oxygen content, the Pr 4f–O 2p hybridization, shortening of c-axis causing Cooper pair-breaking, and the loss of orthorhombicity.

Vortex generation induced by low-frequency wire vibration in superfluidH3e-B

We report the generation of a quantized vortex in superfluid $^{3}\text{H}\text{e-B}$. Oscillating objects can generate vortices due to breaking of Cooper pairs in superfluid $^{3}\text{H}\text{e}$, resulting that the threshold velocity for vortex generation is higher than the pair-breaking velocity. Using very-low-frequency vibrating wires with smooth surfaces, however, we find that the threshold velocity of vortex generation is lower than the pair-breaking velocity. The threshold velocity remains constant with decreasing $^{3}\text{H}\text{e}$ pressure until the pair-breaking velocity equals the vortex-generation threshold velocity at a low pressure. These results indicate that another mechanism that is independent of pressure generates vortices for the low-frequency vibrating wire in superfluid $^{3}\text{H}\text{e}$. The threshold velocity observed in experiments turns out to be nearly equal to the velocity of the instability of the vortex lines attached to the wire, which is characterized by only the circulation quantum.

Microstructural and Superconducting Properties of YBa2Cu3-xCoxO7-δ System

Bulk superconductor samples of YBa2Cu3-xCoxO7-δ with x = 0, 0.01, 0.03, 0.05 are synthesized by solid-state reaction route. Both x-ray diffraction and electron microscopy have been employed to study the phase identification, intergrowths, dislocations and the local structure of these samples. Transition temperature of the samples has been determined by four probe resistivity measurements. The x-ray diffraction patterns indicate that the gross structure/phase of YBa2Cu3-xCoxO7-δ do not change with the substitution of Co up to x = 0.05. The zero resistance critical transition temperature [Tc(R = 0)] is found to decrease and critical current density (Jc) increases with the increased concentration of cobalt in the compound. The Jc enhancement for the cobalt doped samples may be resulting due to flux pinning from some defects such as planar defects, stacking faults and micro defects (twin, domains etc.) and the rapid suppression in Tc may be due to the cooper pair breaking and the hole filling in the CuO2 planes.

Ultrafast structural changes in SrTiO3 due to a superconducting phase transition in a YBa2Cu3O7 top layer

We investigate the structural response of SrTiO3 when Cooper pairs are broken in an epitaxially grown YBa2Cu3O7 top layer due to both heating and optical excitation. The crystal structure is investigated by static, temperature-dependent and time-resolved x-ray diffraction. In the static case, a large strain field in SrTiO3 is formed in the proximity of the onset of the superconducting phase in the top layer, suggesting a relationship between both effects. For the time-dependent studies, we likewise find a large fraction of the probed volume of the SrTiO3 substrate strained if the top layer is superconducting. Upon optical breaking of Cooper pairs, the observed width of the rocking curve is reduced and its position is slightly shifted towards smaller angles. The dynamical theory of x-ray diffraction is used to model the measured rocking curves. We find that the thickness of the strained layer is reduced by about 200 nm on a sub-ps to ps timescale, but the strain value at the interface between SrTiO3 and YBa2Cu3O7 remains unaffected.

Neutral weak currents in nucleon superfluid Fermi liquids: Larkin-Migdal and Leggett approaches

Neutrino emission in the processes of breaking and formation of nucleon Cooper pairs is calculated in the framework of the Larkin-Migdal and Leggett approaches to the description of superfluid Fermi liquids at finite temperatures. We explain peculiarities of both approaches and explicitly demonstrate that they lead to the same expression for the emissivity in pair breaking and formation processes.