SciCombinator

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Concept: Heike Kamerlingh Onnes

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Recently, superconductivity was found on semiconductor surface reconstructions induced by metal adatoms, promising a new field of research where superconductors can be studied from the atomic level.Here we measure the electron transport properties of the Si(111)-(¿7 × ¿3)-In surface near the resistive phase transition and analyze the data in terms of theories of two-dimensional (2D) superconductors.In the normal state, the sheet resistances (2D resistivities) R¿ of the samples decrease significantly between 20 and 5 K, suggesting the importance of the electron-electron scattering in electron transport phenomena.The decrease in R¿ is progressively accelerated just above the transition temperature (Tc) due to the direct (Aslamazov-Larkin term) and the indirect (Maki-Thompson term) superconducting fluctuation effects.A minute but finite resistance tail is found below Tc down to the lowest temperature of 1.8 K, which may be ascribed to a dissipation due to free vortex flow.The present study lays the ground for a future research aiming to find new superconductors in this class of materials.

Concepts: Quantum mechanics, Fundamental physics concepts, Condensed matter physics, Phase transition, Superconductivity, Electrical resistance, Heike Kamerlingh Onnes, History of superconductivity

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Superconductivity is a fascinating quantum phenomenon characterized by zero electrical resistance and the Meissner effect. To date, several distinct families of superconductors (SCs) have been discovered. These include three-dimensional (3D) bulk SCs in both inorganic and organic materials as well as two-dimensional (2D) thin film SCs but only in inorganic materials. Here we predict superconductivity in 2D and 3D organic metal-organic frameworks by using first-principles calculations. We show that the highly conductive and recently synthesized Cu-benzenehexathial (BHT) is a Bardeen-Cooper-Schrieffer SC. Remarkably, the monolayer Cu-BHT has a critical temperature (Tc) of 4.43 K while Tc of bulk Cu-BHT is 1.58 K. Different from the enhanced Tc in 2D inorganic SCs which is induced by interfacial effects, the Tc enhancement in this 2D organic SC is revealed to be the out-of-plane soft-mode vibrations, analogous to surface mode enhancement originally proposed by Ginzburg. Our findings not only shed new light on better understanding 2D superconductivity, but also open a new direction to search for SCs by interface engineering with organic materials.

Concepts: Condensed matter physics, Superconductivity, Electrical resistance, BCS theory, Heike Kamerlingh Onnes, History of superconductivity, London penetration depth, Meissner effect

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Using a microelectromechanical systems (MEMS)-based Fab-on-a-Chip, we quench-condense lead thin-films. Suppressing the formation of lead islands makes it possible to grow a homogeneous and continuous film as thin as 2 nm, without the use of an adhesion layer. Thermal cycling from 3 K to as low as 10 K reveals irreversible annealing of the thin-film characteristic of a metastable state. The transition to the stable state is smooth and is completed by cycling the temperature above ∼42 K, where a distinctive resistance minimum is observed. This resistive minimum is accompanied by an unexpected peak in the superconducting transition temperature. After further thermal cycling, the standard metallic/superconductive behavior is established. The MEMS-based approach yields a platform for systematic studies of quench-condensed thin-film materials, making an intriguing parameter space of mesoscopic physics experimentally accessible.

Concepts: Chemical properties, Semiconductor, Silver, Superconductivity, Thin film, Electrical resistance, Resistivity, Heike Kamerlingh Onnes

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Rb<sub>2</sub>Cr<sub>3</sub>As<sub>3</sub> is a structurally one-dimensional superconductor containing Cr<sub>3</sub>As<sub>3</sub> chains with a superconducting transition temperature of <i>T</i><sub>c</sub> = 4.8 K. Here we report the electrical resistance measurements for Rb<sub>2</sub>Cr<sub>3</sub>As<sub>3</sub> single crystals, under magnetic fields up to 29.5 Tesla and at temperatures down to 0.36 K, from which the upper critical fields, <i>H</i><sub>c2</sub>(<i>T</i>), can be obtained in a broad temperature range. For field parallel to the Cr<sub>3</sub>As<sub>3</sub> chains, <i>H</i><sub>c2</sub><sup>∥</sup>(<i>T</i>), is paramagnetically limited with an initial slope of μ<sub>0</sub>d<i>H</i><sub>c2</sub><sup>∥</sup>/d<i>T</i>│<sub><i>T</i><sub>c</sub></sub> = -16 T/K and a zero-temperature upper critical field of μ<sub>0</sub><i>H</i><sub>c2</sub><sup>∥</sup>(0) = 17.5 T. For field perpendicular to the Cr<sub>3</sub>As<sub>3</sub> chains, however, <i>H</i><sub>c2</sub><sup>⊥</sup>(<i>T</i>) is <i>only</i> limited by orbital pair-breaking effect with μ<sub>0</sub>d<i>H</i><sub>c2</sub><sup>⊥</sup>/d<i>T</i>│<sub><i>T</i><sub>c</sub></sub> = -3 T/K. As a consequence, the anisotropy, γ<sub>H</sub> = <i>H</i><sub>c2</sub><sup>∥</sup>/<i>H</i><sub>c2</sub><sup>⊥</sup>, decreases sharply near <i>T</i><sub>c</sub>, and it reverses below 2 K. Remarkably, the low-temperature <i>H</i><sub>c2</sub><sup>⊥</sup>(T) down to 0.075<i>T</i><sub>c</sub> remains to increase linearly up to over three times of the Pauli paramagnetic limit, which strongly suggests dominant spin-triplet superconductivity in Rb<sub>2</sub>Cr<sub>3</sub>As<sub>3</sub>.

Concepts: Electron, Magnetic field, Fundamental physics concepts, Superconductivity, Electrical resistance, Absolute zero, Heike Kamerlingh Onnes, History of superconductivity

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A reentrant temperature dependence of the normal state resistance often referred to as the N-shaped temperature dependence, is omnipresent in disordered superconductors - ranging from high-temperature cuprates to ultrathin superconducting films - that experience superconductor-to-insulator transition. Yet, despite the ubiquity of this phenomenon its origin still remains a subject of debate. Here we investigate strongly disordered superconducting TiN films and demonstrate universality of the reentrant behavior. We offer a quantitative description of the N-shaped resistance curve. We show that upon cooling down the resistance first decreases linearly with temperature and then passes through the minimum that marks the 3D-2D crossover in the system. In the 2D temperature range the resistance first grows with decreasing temperature due to quantum contributions and eventually drops to zero as the system falls into a superconducting state. Our findings demonstrate the prime importance of disorder in dimensional crossover effects.

Concepts: Quantum mechanics, Fundamental physics concepts, Thermodynamics, Superconductivity, Electrical resistance, Resistivity, Heike Kamerlingh Onnes, History of superconductivity

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The topic of superconductivity in strongly disordered materials has attracted significant attention. These materials appear to be rather promising for fabrication of various nanoscale devices such as bolometers and transition edge sensors of electromagnetic radiation. The vividly debated subject of intrinsic spatial inhomogeneity responsible for the non-Bardeen-Cooper-Schrieffer relation between the superconducting gap and the pairing potential is crucial both for understanding the fundamental issues of superconductivity in highly disordered superconductors, and for the operation of corresponding nanoelectronic devices. Here we report an experimental study of the electron transport properties of narrow NbN nanowires with effective cross sections of the order of the debated inhomogeneity scales. The temperature dependence of the critical current follows the textbook Ginzburg-Landau prediction for the quasi-one-dimensional superconducting channel I c ∼ (1-T/T c)(3/2). We find that conventional models based on the the phase slip mechanism provide reasonable fits for the shape of R(T) transitions. Better agreement with R(T) data can be achieved assuming the existence of short ‘weak links’ with slightly reduced local critical temperature T c. Hence, one may conclude that an ‘exotic’ intrinsic electronic inhomogeneity either does not exist in our structures, or, if it does exist, it does not affect their resistive state properties, or does not provide any specific impact distinguishable from conventional weak links.

Concepts: Electron, Energy, Condensed matter physics, Superconductivity, Electrical resistance, Nanoelectronics, Superconducting magnet, Heike Kamerlingh Onnes

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Competition between superconductivity and disorder plays an essential role in understanding the metal-insulator transition. Based on the Bogoliubov-de Gennes framework, we studied an 2D s-wave fermionic optical lattice system with both spin- orbit coupling and disorder are presented. We find that, with the increase of the strength of disorder, the mean superconducting order parameter will vanish while the energy gap will persist, which indicates that the system undergoes a transition from a superconducting state to a gapped insulating state. This can be confirmed by calculating the inverse participation ratio. We also find that, if the strength of disorder is small, the superconducting order parameter and the energy gap will decrease if we increase the strength of spin-orbit coupling and Zeeman field. In the large disorder limits, the increase of the strength of spin- orbit coupling will increase the mean superconducting order parameter. This phenomenon shows that the system is more insensitive to disorder if the spin-orbit coupling is presented. Numerical computing also shows that the whole system breaks up into several superconducting islands instead of being superconductive.

Concepts: Electron, Fundamental physics concepts, Condensed matter physics, Quantum field theory, Thermodynamics, Superconductivity, Absolute zero, Heike Kamerlingh Onnes

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Surface atomic-layer (SAL) superconductors consisting of epitaxially grown metal adatoms on a clean semiconductor surface have been recently established. Compared to conventional metal thin films, they have two important features: (i) space-inversion symmetry-breaking throughout the system and (ii) high sensitivity to surface adsorption of foreign species. These potentially lead to manifestation of the Rashba effect and a Zeeman field exerted by adsorbed magnetic organic molecules. After introduction of the archetypical SAL superconductor Si(111)-(√7 × √3)-In, we describe how these features are utilized to engineer a topological superconductor with Majorana fermions and discuss its promises and expected challenges.

Concepts: Electron, Fundamental physics concepts, Quantum field theory, Atom, Adsorption, Materials science, Cooper pair, Heike Kamerlingh Onnes

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High-Tc superconductors confined to two dimension exhibit novel physical phenomena, such as superconductor-insulator transition. In the Bi2Sr2CaCu2O8+x (Bi2212) model system, despite extensive studies, the intrinsic superconducting properties at the thinness limit have been difficult to determine. Here, we report a method to fabricate high quality single-crystal Bi2212 films down to half-unit-cell thickness in the form of graphene/Bi2212 van der Waals heterostructure, in which sharp superconducting transitions are observed. The heterostructure also exhibits a nonlinear current-voltage characteristic due to the Dirac nature of the graphene band structure. More interestingly, although the critical temperature remains essentially the same with reduced thickness of Bi2212, the slope of the normal state T-linear resistivity varies by a factor of 4-5, and the sheet resistance increases by three orders of magnitude, indicating a surprising decoupling of the normal state resistance and superconductivity. The developed technique is versatile, applicable to investigate other two-dimensional (2D) superconducting materials.

Concepts: Electron, Quantum mechanics, Fundamental physics concepts, Condensed matter physics, Superconductivity, Electrical resistance, Resistivity, Heike Kamerlingh Onnes

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Temperature-dependent electronic transport in the vicinity of the superconducting transition is reported for quasi-two-dimensional textured FeSe thin films. The conspicuous rounding of the resistive transitions and large transition widths are indications of excess conductivity due to thermal Cooper-pair fluctuations, which can be well-described by the two-dimensional Aslamazov-Larkin theory. The Halperin-Nelson form of the sheet resistance between the phase-ordering temperature TBKT and the mean-field temperature TMF, and the power-law behaviour of the voltage-current characteristics, with a distinctive jump of the exponent at TBKT, are indicative of a Berezinskii-Kosterlitz-Thouless transition. The complementary results suggest a two-dimensional character of superconductivity in the FeSe films and allow a quantitative estimate of the Ginzburg number Gi.

Concepts: Semiconductor, Superconductivity, Electrical resistance, Resistivity, Absolute zero, Sheet resistance, Heike Kamerlingh Onnes, History of superconductivity