用聚焦离子束工艺形成碳化钨：一种获得高磁场弹性图案化超导纳米结构的途径
Formation of tungsten carbide by focused ion beam process: A route to high magnetic field resilient patterned superconducting nanostructures
摘要：
超导体的磁场弹性的尺度由顺磁极限设定。比较Bardeen-Cooper-Schrieffer（BCS）单重态基态与顺磁极化态的凝聚能表明，对于外加场${μ_0}H > 1.8~T_c$（SI），单重态配对在能量上是不利的。从基础和技术的角度来看，超过或接近这个极限的材料是有趣的。这可能是三重态超导性、Fulde-Ferrell-Larkin-Ovchinnikov（FFLO）配对和涉及表面状态的拓扑方面的其他机制的潜在指标，并且还允许在高磁场下的库珀对注入。我们已经分析了这种材料的微观成分，这种材料来自一个意想不到的来源。有机金属气体$\rm {W[(CO)_6]}$的微射流可以被镓离子束分解，留下具有显著超导性质的镓、钨和碳的复杂残留物的轨迹，如高于其顺磁极限的上临界场$H_{c2} > 10~{\rm T} $。我们进行了原子探针断层扫描，以建立纳米晶碳化钨（WC）的轨道和自由钨的情况下形成。支持性计算表明，对于分布在WC表面的Ga，其s，p轨道增强了费米能级附近的态密度。观察到的$H_{c2}(T)$的变化并没有显示出由于粒度而导致的临界场增强的典型特征。我们的观察结果可能是有意义的背景下，一些最近的理论计算的能带结构的WC和实验观察的超导电性在WC-金属界面。
英文摘要：
A scale for magnetic field resilience of a superconductor is set by the paramagnetic limit. Comparing the condensation energy of the Bardeen-Cooper-Schrieffer (BCS) singlet ground state with the paramagnetically polarised state suggests that for an applied field ${μ_0}H > 1.8~T_c$ (in SI), singlet pairing is not energetically favourable. Materials exceeding or approaching this limit are interesting from fundamental and technological perspectives. This may be a potential indicator of triplet superconductivity, Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) pairing and other mechanisms involving topological aspects of surface states, and also allow Cooper pair injection at high magnetic fields. We have analysed the microscopic composition of such a material arising from an unexpected source. A microjet of an organo-metallic gas, $\rm {W[(CO)_6]}$ can be decomposed by gallium ion-beam, leaving behind a track of complex residue of gallium, tungsten and carbon with remarkable superconducting properties, like an upper critical field, $H_{c2} > 10~{\rm T} $, above its paramagnetic limit. We carried out Atomic probe tomography to establish the formation of nano-crystalline tungsten carbide (WC) in the tracks and the absence of free tungsten. Supporting calculations show for Ga distributed on the surface of WC, its s,p-orbitals enhance the density of states near the Fermi energy. The observed variation of $H_{c2}(T)$ does not show features typical of enhancement of critical field due to granularity. Our observations may be significant in the context of some recent theoretical calculation of the band structure of WC and experimental observation of superconductivity in WC-metal interface.
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