Phase slips are topological fluctuation events that carry the superconducting order-parameter field between distinct current carrying states 1 . Owing to these phase slips low-dimensional superconductors acquire electrical resistance 2 . In quasi-one-dimensional nanowires it is well known that at higher temperatures phase slips occur via the process of thermal barrier-crossing by the orderparameter field. At low temperatures, the general expectation is that phase slips should proceed via quantum tunnelling events, which are known as quantum phase slips (QPS). However, resistive measurements have produced evidence both pro 3-6 and con [7][8][9] and hence the precise requirements for the observation of QPS are yet to be established firmly. Here we report strong evidence for individual quantum tunnelling events undergone by the superconducting order-parameter field in homogeneous nanowires. We accomplish this via measurements of the distribution of switching currents-the high-bias currents at which superconductivity gives way to resistive behaviour-whose width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. We outline a Quantum phenomena involving systems far larger than individual atoms are one of the most exciting fields of modern physics. Initiated by Leggett more than twentyfive years ago 14,15 , the field has seen widespread development, important realizations being furnished, e. g., by macroscopic quantum tunnelling (MQT) of the phase in Josephson junctions, and of the magnetization in magnetic nanoparticles [16][17][18][19] . More recently, the breakthrough recognition of the potential advantages of quantum-based computational methods has initiated the search for viable implementations of qubits 20 , several of which are rooted in MQT in superconducting systems. In particular, it has been recently proposed that superconducting nanowires (SCNWs) could provide a valuable setting for realizing qubits 12 . In this case, the essential behaviour needed of SCNWs that they undergo QPS, i.e., topological quantum fluctuations of the superconducting order-parameter field via which tunnelling occurs between currentcarrying states. It has also been proposed that QPS in nanowires could allow one to build a current standard, and thus could play a useful role in aspects of metrology 13 .Additionally, QPS are believed to provide the pivotal processes underpinning the 3 superconductor-insulator transition observed in nanowires 21-25, Observations of QPS have been reported previously on wires having high normal resistance (i.e., R N > R Q , where R Q = h/4e 2 ≈ 6,450 Ω) via low-bias resistance (R) vs. temperature (T) measurements 3,4 . Yet, low-bias measurements on short wires with normal resistance R N < R Q have been unable to reveal QPS 7,8 . Also, it has been suggested that some results ascribed to QPS could in fact have originated in inhomogeneity of the nanowires.Thus, no consensus exists about the conditions under which QPS occur, and qualitatively new evidence for QPS remains highl...

Large sensitive-area NbN nanowire superconducting single-photon detectors fabricated on single-crystal MgO substrates Appl. Phys. Lett. 92, 061116 (2008); 10.1063/1.2870099Effect of heating rates on superconducting properties of pure MgB 2 , carbon nanotube-and nano-SiC-doped in situ MgB 2 ∕ Fe wires Continuous Nb wires, 7-15 nm in diameter, have been fabricated by sputter-coating single fluorinated carbon nanotubes. Transmission electron microscopy revealed that the wires are polycrystalline, having grain sizes of about 5 nm. The critical current of wires thicker than ϳ12 nm is very high (10 7 A/cm 2 ) and comparable to the expected depairing current. The resistance versus temperature curves measured down to 0.3 K are well described by the Langer-Ambegaokar-McCumber-Halperin theory of thermally activated phase slips. Quantum phase slips are suppressed.

The effects of strong magnetic field on superconducting Nb and MoGe nanowires with diameter ∼ 10 nm have been studied. We have found that the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory of thermally activated phase slips is applicable in a wide range of magnetic fields and describes well the temperature dependence of the wire resistance, over eleven orders of magnitude. The field dependence of the critical temperature, Tc, extracted from the LAMH fits is in good quantitative agreement with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions. The extracted spin-orbit scattering time agrees with an estimate τso ≃ τ ( c/Ze 2 ) 4 , where τ is the elastic scattering time and Z is the atomic number.PACS numbers: 74.78. Na, 74.25.Fy, 74.25.Ha, 74.40.+k The problem of superconductivity in one-dimensional (1D) systems attracts much attentions since it involves such fundamental phenomena as macroscopic quantum tunnelling, quantum phase transitions and environmental effects [1,2,3,4,5,6,7]. It is expected that a strong magnetic field can be used to control these phenomena. Indeed, the microscopic theory predicts that a magnetic field, acting on a superconducting condensate, lifts the time reversal symmetry of the spin and orbital states of paired electrons and suppresses the critical temperature, T c [8,9]. A strong enough field destroys superconductivity. The magnetic field pair-breaking effects were studied in depth in two and zero-dimensional systems, i.e. thin films [10] and nanograins [11]. However, an experimental verification of the pair breaking effects in 1D superconductors is long overdue.A distinct feature of 1D superconductors is the absence of the phase coherence. Due to fluctuations the amplitude of the order parameter has a finite probability to reach zero at some point along the wire, allowing the phase of the order parameter to slip by 2π [12]. The theory of thermally activated phase slips (TAPS) was developed by Langer, Ambegaokar, McCumber and Halperin (LAMH). However the effect of the magnetic field on the phase slippage process is not established. It is also unknown whether the magnetic field can change the relative contributions of quantum and thermally activated phase slips in thin wires [3,4].In this Letter we study the effects of the magnetic field on the phase slippage rate and the critical temperature of thin wires. It is found that the LAMH provides a good description for 1D superconductors in magnetic fields up to 11 T. The dependence of the critical temperature on the magnetic field, T c (B) agrees well with the theory of pair-breaking perturbations that takes into account both spin and orbital contributions [8,9]. This is our main result. No significant contribution of quantum phase slips has been detected in the studied samples.The samples were made by sputter-coating of suspended fluorinated carbon nanotubes with Nb or Mo 79 Ge 21 . Transport measurements were performed in a He-3 cryostat, as described previously [2,4,5]. The magnet...

We study the effect of an applied magnetic field on sub-10nm wide MoGe and Nb superconducting wires. We find that magnetic fields can enhance the critical supercurrent at low temperatures, and does so more strongly for narrower wires. We conjecture that magnetic moments are present, but their pair-breaking effect, active at lower magnetic fields, is suppressed by higher fields. The corresponding microscopic theory, which we have developed, quantitatively explains all experimental observations, and suggests that magnetic moments have formed on the wire surfaces.