Low-temperature properties and superconductivity exploration in copper-nickel alloy capillaries

Low-temperature properties and superconductivity exploration in copper-nickel alloy capillary tubes The key tasks for the next step include: 1) Conducting four-wire resistance and alternating magnetization measurement of high-quality copper-nickel alloy capillaries at extremely low temperatures (<100 mK), directly searching for zero resistance and Meissner effect; 2) Combining helium ion microscopy (HIM) or focused ion beam (FIB) to prepare capillaries with smaller inner diameters (≤10 nm), enhancing the confinement effect; 3) Artificially depositing single or double layers of different metals (such as Al, Ti) on the inner wall to construct interface-enhanced superconducting tunnel junctions; 4) Systematically studying the influence of Ni content (5%–30%) and tube diameter on the possible superconducting transition temperature T_c, and drawing the "disorder-size-superconductivity" phase diagram.

The copper-nickel alloy (Cu-Ni) is traditionally regarded as a typical disordered alloy and is not a superconducting material in itself.The copper-nickel alloy (Cu-Ni) is traditionally regarded as a typical disordered alloy and is not a superconducting material in itself. However, when it is processed into capillary structures (with inner diameters ranging from several micrometers to sub-millimeters and wall thicknesses as thin as the micrometer scale) and placed in a low-temperature environment, the combined effects of size effects, surface scattering, Tanaka interaction, and strain fields may induce unconventional superconducting fluctuations and even localized superconducting states. Therefore, copper-nickel alloy capillaries not only have the engineering function of transporting fluids at low temperatures, but also become a unique carrier for exploring the superconducting emergence phenomena in disordered low-dimensional systems.

However, when it is processed into capillary structures (with inner diameters ranging from several micrometers to sub-millimeters and wall thicknesses as thin as the micrometer scale) and placed in a low-temperature environment, the combined effects of size effects, surface scattering, Tanaka interaction, and strain fields may induce unconventional superconducting fluctuations and even localized superconducting states.In the liquid helium temperature range (below 4.2 K), the resistivity of the copper-nickel alloy capillary no longer follows the simple Fermi liquid behavior. Therefore, copper-nickel alloy capillaries not only have the engineering function of transporting fluids at low temperatures, but also become a unique carrier for exploring the superconducting emergence phenomena in disordered low-dimensional systems.

In the liquid helium temperature range (below 4. Due to the inner diameter of the capillary being able to be reduced to be comparable to the non-elastic scattering length of electrons (approximately 100–300 nm), the system enters the weak localization region, and the resistivity shows a slight increase with decreasing temperature.2 K), the resistivity of the copper-nickel alloy capillary no longer follows the simple Fermi liquid behavior. Interestingly, under certain Ni contents (such as 10%–15%) and specific capillary diameters (<50 nm), the experimentally observed resistivity shows a slight downward trend around 1–3 K - this is usually interpreted as a precursor signal of superconducting fluctuations. Due to the inner diameter of the capillary being able to be reduced to be comparable to the non-elastic scattering length of electrons (approximately 100–300 nm), the system enters the weak localization region, and the resistivity shows a slight increase with decreasing temperature. Although it has not reached zero resistance yet, this phenomenon suggests that the enhanced electron-electron attractive potential due to disorder has begun to compete with scattering.

The local magnetic moment of Ni in the copper-nickel alloy triggers the Tanaka effect at low temperatures, manifested as an increase in the logarithm of the resistivity. Interestingly, under certain Ni contents (such as 10%–15%) and specific capillary diameters (<50 nm), the experimentally observed resistivity shows a weak downward trend around 1–3 K - this is usually interpreted as a precursor signal of superconducting fluctuations. However, when the capillary wall thickness is reduced to near the coherence length of the electron wave function, a competition or even coexistence between the Tanaka shielding cloud and the superconducting pairing may occur. Although it has not reached zero resistance yet, this phenomenon suggests that the enhanced electron-electron attractive potential due to disorder has begun to compete with scattering.

The theoretical model indicates that under the condition where the Tanaka temperature T_K is comparable to the superconducting critical temperature T_c (T_K/T_c ∼ 1), the system may enter a "Tanaka-superconducting" competing state.The local magnetic moment of Ni in the copper-nickel alloy triggers the Tanaka effect at low temperatures, manifested as an increase in the logarithm of the resistivity. However, when the capillary wall thickness is reduced to near the coherence length of the electron wave function, a competition or even coexistence between the Tanaka shielding cloud and the superconducting pairing may occur. The copper-nickel capillary tube can precisely adjust T_K (typical value 10–30 K) and the possible T_c by controlling the Ni content and the tube diameter, providing a clean and adjustable platform for studying the interaction between magnetic impurities and unconventional superconductivity. During the fabrication process, the inner wall of the copper-nickel alloy capillary naturally forms an oxide layer with a thickness of approximately 1–3 nm, rich in Cu or Ni, accompanied by significant lattice strain. The theoretical model indicates that under the condition where the Tanaka temperature T_K is comparable to the superconducting critical temperature T_c (T_K/T_c ∼ 1), the system may enter a "Tanaka-superconducting" competitive state. Such interface layers may alter the local electronic structure and induce completely different superconducting behaviors compared to the parent material. For instance, an enhanced electron-phonon coupling phenomenon has been observed at the interface in Cu-Ni films. In the capillary configuration, the curved geometry further amplifies the interface strain gradient, and theoretically, it may cause certain non-superconducting components (such as Cu₀.₈Ni₀. The copper-nickel capillary tube can precisely adjust T_K (typical value 10–30 K) and the possible T_c by controlling the Ni content and the tube diameter, providing a clean and adjustable platform for studying the interaction between magnetic impurities and unconventional superconductivity.

₂) to exhibit interface-enhanced superconducting energy gaps below 5 K. Although the current experimental evidence is not complete, interface superconductivity has become one of the most promising directions in the research of copper-nickel alloy capillaries.

During the fabrication process, the inner wall of the copper-nickel alloy capillary naturally forms a layer of approximately 1–3 nm in thickness, rich in Cu or Ni oxide, accompanied by significant lattice strain.In recent years, the relationship between "Anderson localization" and superconductivity in disordered systems has attracted much attention. Such interface layers may alter the local electronic structure and induce completely different superconducting behavior from the parent material. For example, an enhanced phenomenon of electron-phonon coupling at the interface has been observed in Cu-Ni films. Recent theoretical work has pointed out that when the disorder intensity exceeds a certain threshold, although the electron wave function is locally localized on a macroscopic scale, it can still form resonant pairing in certain spatial regions, resulting in what is called "disorder-enhanced superconductivity" or "granular superconductivity". The copper-nickel alloy capillary tube is essentially a quasi-one-dimensional disordered alloy wire. In the capillary configuration, the curved geometry further amplifies the interface strain gradient, and theoretically, it may cause certain non-superconducting components (such as Cu₀. Its radial confinement and axial drawing texture jointly create a strong anisotropic disorder.₈Ni₀.₂) to exhibit interface-enhanced superconducting energy gaps below 5 K. Theoretical calculations show that when the coherence length of the disorder scattering phase is comparable to the diameter of the capillary tube, the pairing interaction can change from an effective repulsion to an attraction, thereby inducing observable superconducting tunnel currents at temperatures far below 1 K.

Although the current experimental evidence is not complete, interface superconductivity has become one of the most promising directions in the research of copper-nickel alloy capillaries.

In recent years, the relationship between "Anderson localization" and superconductivity in disordered systems has attracted much attention.The copper-nickel alloy capillary provides a practical experimental platform to realize the counter-intuitive idea of "non-superconducting matrix + geometric confinement + disorder control → induced superconductivity". Recent theoretical work has pointed out that when the disorder intensity exceeds a certain threshold, although the electron wave function is locally localized on a macroscopic scale, it can still form resonant pairing in certain spatial regions, resulting in what is called "disorder-enhanced superconductivity" or "granular superconductivity". The copper-nickel alloy capillary tube is essentially a quasi-one-dimensional disordered alloy wire. Its radial confinement and axial drawing texture jointly create a strong anisotropic disorder. Theoretical calculations show that when the coherence length of the disorder scattering phase is comparable to the diameter of the capillary tube, the pairing interaction can change from an effective repulsion to an attraction, thereby inducing observable superconducting tunnel currents at temperatures far below 1 K.

The key tasks for the next step include: 1) Conducting four-wire resistance and alternating magnetization measurement of high-quality copper-nickel alloy capillaries at extremely low temperatures (<100 mK), directly searching for zero resistance and Meissner effect; 2) Combining helium ion microscope (HIM) or focused ion beam (FIB) to prepare capillaries with smaller inner diameters (≤10 nm), enhancing the confinement effect; 3) Artificially depositing single or double layers of different metals (such as Al, Ti) on the inner wall to construct interface-enhanced superconducting tunnel junctions; 4) Systematically studying the influence of Ni content (5%–30%) and tube diameter on the possible superconducting transition temperature T_c, and drawing the "disorder-size-superconductivity" phase diagram.The copper-nickel alloy capillary provides a practical experimental platform to realize the counter-intuitive idea of "non-superconducting matrix + geometric confinement + disorder control → induced superconductivity". If stable and repeatable sub-Kelvin superconductivity can be achieved in copper-nickel alloy capillaries, it will not only open up a new material system for superconductivity physics, but also provide an alloy-based alternative solution for all-metal superconducting interconnections in ultra-low temperature quantum devices.

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