Cooper Pairs Research Articles

On the Thermodynamic Properties of FexSe0.5Te0.5 Superconducting Single Crystals: An Experimental Study

The variation of the thermodynamic critical fields (Hc) with temperature has been obtained from the equilibrium magnetization of FexSe0.5Te0.5 superconducting single-crystal with (0.95 ≤ × ≤ 1.1). Based on the BCS theory, we used the Hc values to obtain the variations in the energy gap Δ(T), the cooper pair coupling, the energy gap ratio 2Δ(0)/kBTc, and the specific heat jump ΔCBCS/Tc with varying Fe content. We found that the Cooper pairs coupling strength decreases with increasing Fe content within the range (∼3.1 to 2.6) for 1.0 ≤ × ≤ 1.1, which is consistent with the BCS-weak coupling limit for all samples. Moreover, the value of the energy gap is nearly constant at about 3.5 meV for all samples, while the Hc(0)/Tc ratio reaches a maximum near x ∼ 1.05. The obtained values of the energy gap, the coupling strength, and the specific heat ratio are consistent with recently published results using various experimental techniques.

Strong interlayer magnetic exchange coupling in La3Ni2O7−δ revealed by inelastic neutron scattering

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La3Ni2O7 under pressure provides a new platform to elucidate the origins of high-temperature superconductivity. We perform elastic and inelastic neutron scattering studies on a polycrystalline sample of La3Ni2O7−δ at ambient pressure. No magnetic order can be identified down to 10 K. The absence of long-range magnetic order in neutron diffraction measurements may be ascribed to the smallness of the magnetic moment. However, we observe a weak flat spin-fluctuation signal in the inelastic scattering spectra at ∼ 45 meV. The observed spin excitations could be interpreted as a result of strong interlayer and weak intralayer magnetic couplings for stripe-type antiferromagnetic orders. Our results provide crucial information on the spin dynamics and are thus important for understanding the superconductivity in La3Ni2O7.

Manipulation of Cooper-Pair Tunneling via Domain Structure in a van der Waals Ferromagnetic Josephson Junction.

Ferromagnetic Josephson junctions play a key role in understanding the interplay between superconductivity and ferromagnetism in condensed matter physics. The magnetic domain structures of the ferromagnet in such junctions can significantly affect the tunneling of the superconducting Cooper pairs due to the strong interactions between Cooper pairs and local magnetic moments in the ferromagnetic tunnel barrier. However, the underlying microscopic mechanism of relevant quasiparticle tunneling processes with magnetic domain structures remains largely unexplored. Here, the manipulation of Cooper-pair tunneling in the NbSe2/Cr2Ge2Te6/NbSe2 ferromagnetic Josephson junction is demonstrated by using a multidomain ferromagnetic barrier with anisotropic magnetic moments. The evolution of up-, down-magnetized domain and Bloch domain structures in Cr2Ge2Te6 barrier under external magnetic fields leads to the enhancement of the critical tunneling supercurrent and an unconventional dual-peak feature with two local maxima in the field-dependent critical current curve. The phenomenon of magnetic-field-modulated critical tunneling supercurrent can be well explained by the competition between the coherence length of tunneling Cooper pairs and the size of magnetic domain walls in Cr2Ge2Te6 barrier. This kind of ferromagnetic Josephson junction provides an intriguing material system for manipulating Cooper-pair tunneling by tuning the local magnetic moments within magnetic Josephson junction devices.

Drag on Cylinders Moving in Superfluid 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He-B as the Dimension Spans the Coherence Length

Vibrating probes when immersed in a fluid can provide powerful tools for characterising the surrounding medium. In superfluid 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He-B, a condensate of Cooper pairs, the dissipation arising from the scattering of quasiparticle excitations from a mechanical oscillator provides the basis of extremely sensitive thermometry and bolometry at sub-millikelvin temperatures. The unique properties of the Andreev reflection process in this condensate also assist by providing a significantly enhanced dissipation. While existing models for such damping on an oscillating cylinder have been verified experimentally, they are valid only for flows with scales much greater than the coherence length of 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He, which is of the order of a hundred nanometres. With our increasing proficiency in fabricating nanosized oscillators, which can be readily used in this superfluid, there is a pressing need for the development of new models that account for the modification of the flow around these smaller oscillators. Here we report preliminary results on measurements of the damping in superfluid 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He-B of a range of cylindrical nanosized oscillators with radii comparable to the coherence length and outline a model for calculating the associated drag.

Imaging momentum-space Cooper pair formation and its competition with the charge density wave gap in a kagome superconductor

Imaging momentum-space Cooper pair formation and its competition with the charge density wave gap in a kagome superconductor

New pairing mechanism via chiral electron–hole condensation for non-BCS superconductivity

A novel chiral electron–hole (CEH) pairing mechanism is proposed to account for non-BCS superconductivity. In contrast to BCS Cooper pairs, CEH pairs exhibit a pronounced affinity to antiferromagnetism for superconductivity. The gap equations derived from this new microscopic mechanism are analyzed for both s- and d-wave superconductivity, revealing marked departures from the BCS theory. Unsurprisingly, CEH naturally describes superconductivity in strongly-correlated systems, necessitating an exceedingly large coupling parameter (λ>1 for s-wave and λ>π/2 for d-wave) to be efficacious. The new mechanism provides a better understanding of various non-BCS features, especially in cuprate and iron-based superconductors. In particular, CEH, through quantitative comparison with experimental data, shows promise in solving long-standing puzzles such as the unexpectedly large gap-to-critical-temperature ratio Δ0/Tc, the lack of gap closure at Tc, superconducting phase diagrams, and a non-zero heat-capacity-to-temperature ratio C/T at T=0 (i.e., the “anomalous linear term”), along with its quadratic behavior near T=0 for d-wave cuprates.

Two-photon emission from a superlattice-based superconducting light-emitting structure

Superconductor-semiconductor hybrid devices can bridge the gap between solid-state-based and photonics-based quantum systems, enabling new hybrid computing schemes, offering increased scalability and robustness. One example for a hybrid device is the superconducting light-emitting diode (SLED). SLEDs have been theoretically shown to emit polarization-entangled photon pairs by utilizing radiative recombination of Cooper pairs. However, the two-photon nature of the emission has not been shown experimentally before. We demonstrate two-photon emission in a GaAs/AlGaAs SLED. Measured electroluminescence spectra reveal unique two-photon superconducting features below the critical temperature (Tc), while temperature-dependent photon-pair correlation experiments (g(2)(τ,T)) demonstrate temperature-dependent time coincidences below Tc between photons emitted from the SLED. Our results pave the way for compact and efficient superconducting quantum light sources and open new directions in light-matter interaction studies.

Ultrafast Terahertz Superconductor Van der Waals Metamaterial Photonic Switch

The high‐temperature layered superconductor (HTS) BSCCO is one of the key quantum material platforms in THz science and technology. Compact, stable, and reliable BSCCO THz photonic integrated circuit components can be developed to effectively and efficiently control and manipulate THz wave radiation, especially for future communication systems and network applications. Herein, the design, simulation, and modeling of ultrafast THz metamaterial photonic integrated circuits are reported on a few nanometer‐thick HTS BSCCO van der Waals (vdWs), capable of the active modulation of phase with constant transmission coefficient over a narrow‐frequency range. Meanwhile, the metamaterial circuit works as an amplitude modulator without significantly changing the phase in a different frequency band. Under the application of ultrashort optical pulses, the transient modulation dynamics of the THz metamaterial offer a fast‐switching timescale of 50 ps. The dynamics of picosecond light–matter interaction, Cooper pairs breaking, photoinduced quasiparticles generation and recombination, phonon bottleneck effect, and emission and relaxation of bosons in BSCCO vdW metamaterial arrays are discussed for the potential application of multifunctional superconducting photonic integrated circuits in communication and quantum technologies.

Superconductivity in Twisted Bismuth Bilayers

AbstractTwo twisted bismuth bilayers, TBB, each with 120 atoms, are studied using their electron density of states and their vibrational density of states using first‐principles calculations. Metallic character at the Fermi level is found for the non‐rotated sample as well as for each sample rotated 0.5°, 1.0°, 1.5°, 2.0°, 2.5°, 3.0°, 4.0°, 5.0°, 6.0°, 7.0°, 8.0°, and 10° with respect to each other. Assuming that the superconductivity is BCS‐type and the invariance of the Cooper pairing potential, a maximum superconducting temperature TC ≈1.8 K is predicted for a magic angle of 0.5° between the two bilayers, increasing the superconducting transition temperature from the experimentally measured value of 0.53 mK for the Wyckoff structure of crystalline bismuth.

Weak and strong chirality-anomaly-manipulations in a superconducting Weyl semimetal sandwich structure

We investigate the quantum interference of the electron–hole conversions from the two interfaces in a Weyl semimetal (WSM)-based hybrid structure, in which a superconducting WSM is sandwiched in between two normal ones. The quantum interference is characterized by the chirality-anomaly-manipulation (CAM). It is found that only low energy is in favor for s-wave BCS pairing states. The Andreev reflection (AR) chirality blockade can be tuned by the stagger angle α for the relative orientation of paired Weyl points, accompanied by an AR bipolar chirality diode. Thus, a strong CAM is indicated for the electron–hole conversion. However, the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) pairing states have no energy preference, with the weak and strong CAMs being near and far away from the zero energy, respectively. More interestingly, a perfect AR with the normal reflection suppressed thoroughly can be obtained at any α as a result of the FFLO paring with the same chirality. In addition, the conductance or noise power, which incorporates the contributions of the two paired Weyl nodes, not only, in turn, embodies the respective features of their contributions but also can be experimentally measured to discern between the BCS and FFLO paring states.

Crossover from Cooper-pair hopping to single-electron hopping in Pb x (TiO2) 1−x granular films

The electrical transport properties of Pb x (TiO2) 1−x (x being the Pb volume fraction and ranging from ∼0.45 to ∼0.72) granular films are investigated experimentally. The charging energy of the Pb granules is reduced to less than the superconducting gap of Pb granules for the low temperature insulating films by using high-k dielectric TiO2 as the insulating matrix. For the insulating films in the vicinity of the superconductor-insulator transition, Cooper-pair hopping governs the low-temperature hopping transport. For these films, the low-temperature magnetoresistance is positive at low field and the resistivity vs temperature for Cooper-pair hopping obeys an Efros–Shklovsii-type variable-range-hopping law. A crossover from Cooper-pair-dominated hopping to single-electron-dominated hopping is observed with decreasing x. The emergence of single-electron-dominated hopping in the more insulating films is due to the causation that the intergrain Josephson coupling becomes too weak for Cooper pairs to hop between adjacent Pb granules.

Magnetic phases of XY model with three-spin terms: interplay of topology and entanglement

Magnetic and topological properties along with quantum correlations in terms of several entanglement measures have been investigated for an antiferromagnetic (AFM) spin-1/2 XY model in the presence of transverse magnetic field and XZX−YZY type of three-spin interactions. Symmetries of the spin Hamiltonian have been identified. Under the Jordan–Wigner transformation, the spin Hamiltonian converted into spinless superconducting model with nearest neighbor (NN) hopping and Cooper pairing terms in addition to next NN Cooper pairing potential. Long range AFM order has been studied in terms of staggered spin–spin correlation functions, while the topological orders have been characterized by winding numbers. Magnetic and topological phase diagrams have been prepared. Faithful coexistence of magnetic and topological superconducting phases is found in the entire parameter regime. Boundaries of various quantum phases have been marked and positions of bicritical points have been identified.

Andreev Conductance in Disordered SF Junctions with Spin-Orbit Scattering

We calculate the conductance of a junction between a disordered superconductor and a very strong half-metallic ferromagnet admitting electrons with only one spin projection. A usual mechanism of Andreev reflection is strongly suppressed in this case since Cooper pairs are composed of electrons with opposite spins. However, this obstacle can be overcome if we take into account spin-orbit scattering inside the superconductor. Spin-orbit scattering induces a fluctuational (zero on average) spin-triplet component of the superconducting condensate, which is enough to establish Andreev transport into a strong ferromagnet. This remarkably simple mechanism is quite versatile and can explain long-range triplet proximity effect in a number of experimental setups. One particular application of the suggested effect is to measure the spin-orbit scattering time τSO\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ au _{\ ext {SO}}$$\\end{document} in disordered superconducting materials. The value of Andreev conductance strongly depends on the parameter ΔτSO\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\Delta \ au _\ ext {SO}$$\\end{document} and can be noticeable even in very disordered but relatively light metals like granular aluminum.

Systematic investigation on semi-empirical thermodynamic quantities of excited nuclei using canonical ensemble

Semi-empirical thermodynamic quantities (TQs) of 78 nuclei ranging from 43Sc to 243Pu have been systematically investigated in the temperature region below 1 MeV using the thermodynamic canonical ensemble. The latter is carried out by taking into account the experimental nuclear level density (NLD) data measured using the Oslo method for the low-excitation region below the neutron binding energy B n combining with the back-shifted Fermi gas NLD model for the excitation energy from B n to about 250 MeV. In particular, the uncertainty of the TQs propagating from the fluctuation of the experimental NLD data has been, for the first time, calculated. The results obtained indicate that the uncertainty of TQs due to the experimental NLD is incomparable with the changes caused by the nuclear structure effects. The free energy of even–even nuclei behaves differently from that of odd-A ones. The total energy in the low-temperature region below T E ≃ 0.4 − 0.6 MeV for medium-mass nuclei and T E ≃ 0.2 − 0.4 MeV for heavy-mass ones slowly varies. When temperature is from T E to 1 MeV, the total energy increases extremely faster than the increase of temperature, exhibiting the constant-temperature behavior. The entropy exhibits an abrupt change in their slope at T S ≃ 0.2 − 0.4 MeV in medium-mass nuclei and T S ≃ 0.5 − 0.6 MeV in heavy-mass ones. The existence of T E and T S has been interpreted due to the breaking of the first Cooper pair. Finally, the heat capacity shows a strongly pronounced S-shape in nuclei belonging to the rare-earth region. The temperatures defined at the center of the S − shaped heat capacities, which are known to closely relate to the critical temperature of the pairing phase transition T C, are quite close to those theoretically predicted, namely T C ≈ 0.5Δ − 0.6Δ with Δ = 12A −1/2 being the empirical pairing gap at zero temperature. The semi-empirical TQs obtained in the present work can be, therefore, a reliable data source to test and/or validate many nuclear thermodynamical models and to examine some nuclear structure properties such as pairing and deformation.

Charge- 4e and Charge- 6e Flux Quantization and Higher Charge Superconductivity in Kagome Superconductor Ring Devices

The flux quantization is a key indication of electron pairing in superconductors. For example, the well-known h/2e flux quantization is considered strong evidence for the existence of charge-2e, two-electron Cooper pairs. Here we report evidence for multicharge flux quantization in mesoscopic ring devices fabricated using the transition-metal kagome superconductor CsV3Sb5. We perform systematic magnetotransport measurements and observe unprecedented quantization of magnetic flux in units of h/4e and h/6e in magnetoresistance oscillations. Specifically, at low temperatures, magnetoresistance oscillations with period h/2e are detected, as expected from the flux quantization for charge-2e superconductivity. We find that the h/2e oscillations are suppressed and replaced by resistance oscillations with h/4e periodicity when the temperature is increased. Increasing the temperature further suppresses the h/4e oscillations, and robust resistance oscillations with h/6e periodicity emerge as evidence for charge-6e flux quantization. Our observations provide the first experimental evidence for the existence of multicharge flux quanta and emergent quantum matter exhibiting higher-charge superconductivity in the strongly fluctuating region above the charge-2e Cooper pair condensate, revealing new insights into the intertwined and vestigial electronic order in kagome superconductors. Published by the American Physical Society 2024

Cooper Pairs Pair Up in a Kagome Metal

Cooper Pairs Pair Up in a Kagome Metal

Controllable odd-frequency Cooper pairs in multisuperconductor Josephson junctions

We consider Josephson junctions formed by multiple superconductors with distinct phases and explore the formation of nonlocal or intersuperconductor pair correlations. We find that the multiple superconductor nature offers an additional degree of freedom that broadens the classification of pair symmetries, enabling nonlocal even- and odd-frequency pairings that can be highly controlled by the superconducting phases and the energy of the superconductors. Specially, when the phase difference between two superconductors is π, their associated nonlocal odd-frequency pairing is the only type of intersuperconductor pair correlations. Finally, we show that these nonlocal odd-frequency Cooper pairs dominate the nonlocal conductance via crossed Andreev reflections, which constitutes a direct evidence of odd-frequency pairing. Published by the American Physical Society 2024

Yu–Shiba–Rusinov states induced by single Fe atoms on reconstructed compound superconductor V[formula omitted]Si

Reconstructed surfaces of the A15-compound superconductor V3Si(100) that are possibly induced by the segregation of bulk impurities serve as platforms to study the dependence of Yu–Shiba–Rusinov states induced by a single Fe atom on the adsorption site. Their number, energy and electron–hole asymmetry vary strongly with the atomic environment of the Fe atom. These variations are indicative of different Fe d-orbitals being active in the site-dependent exchange coupling with the substrate Cooper pairs. Spatially resolved spectroscopy gives rise to a short decay length of the Yu–Shiba–Rusinov states and thereby suggests the three-dimensional character of the scattering process underlying the bound states.

Cooper pairing, flat-band superconductivity, and quantum geometry in the pyrochlore-Hubbard model

Cooper pairing, flat-band superconductivity, and quantum geometry in the pyrochlore-Hubbard model

RKKY interaction in triplet superconductors: Dzyaloshinskii-Moriya-type interaction mediated by spin-polarized Cooper pairs

RKKY interaction in triplet superconductors: Dzyaloshinskii-Moriya-type interaction mediated by spin-polarized Cooper pairs