We have developed an atomic beam technique for studying magnetic vortices and vortex lattices of superconductors. Atoms moving near a superconductor's surface see a fluctuating magnetic field as they pass vortices. This field may drive magnetic resonance transitions between hyperfine states. Measuring the magnetic resonance transition probability as a function of atom velocity probes the vortex lattice autocorrelation function. We demonstrate this technique by studying the vortex lattice of a niobium film sample and measuring the sample's penetration depth. We also identify a systematic problem that we think thwarted an earlier attempt to experimentally realize this technique.
We have developed an atomic beam technique for studying magnetic vortices and vortex lattices of superconductors. Atoms moving near a superconductor's surface see a fluctuating magnetic field as they pass vortices. This field may drive magnetic resonance transitions between hyperfine states. Measuring the magnetic resonance transition probability as a function of atom velocity probes the vortex lattice autocorrelation function. We demonstrate this technique by studying the vortex lattice of a niobium film sample and measuring the sample's penetration depth. We also identify a systematic problem that we think thwarted an earlier attempt to experimentally realize this technique.
Vortices and vortex lattices play a major role in determining the transport properties of type-II superconductors[1–3], and enable a platform to investigate exotic superconducting physics[4,5]. The study of vortex matter has generally focused on novel states in 2D films and structures, and has recently moved to investigating systems with constrained dimensions and smaller vortex numbers[6–11]. Vortices are responsible, for example, for some electrical transport regimes in superconducting films, as well as the Berezinkskii-Kosterlitz-Thouless phase transition in superconducting films[12]. Unconventional forms of superconductivity, such as the spin triplet pairing predicted in Sr2RuO4, or in topological insulators paired to s-wave superconductors, contain two condensates that may support two vortex lattices, and may display Majorana modes, signatures of which may have been seen in other superconducting systems[13–17]. The vortex-vortex interactions, or inter and intra-condensate couplings in multicondensate systems, are important parameters that characterize the behavior of the systems that display such phenomena[18–20]. In investigating these parameters, a technique that can both probe the energies in a system, as well as manipulate the vortices therein, has long been desired. In this work, we report on progress in determining the energy scales of vortex systems, as well as limited control over the vortex motion. Using a technique based on magnetic force microscopy, we can directly measure the resonant motion of vortices present in a superconducting lattice. We use a scanning magnetic tip to trap a small number of vortices in a superconducting Josephson junction array near the tip. By observing the resonant motion of the configuration of vortices, a map of the location of energy degeneracies between different stable configurations is generated. From this data, we use a simulation to extract the relative strengths of the characteristic energy scales for the system, including the vortex-magnetic field interaction, the vortex-vortex interaction strength, and the chemical potential for the vortices. The simulations for small numbers of vortices fits the data well for multiple field profiles and lattice spacings. The ability to tune the vortex number and configurations by changing the magnetic field profile from the tip, as well as the lattice parameters of the superconducting surface, are key portions of this technique. We demonstrate that the relative strengths of the chemical potential and vortex-vortex interactions can be tuned relative to the vortex-magnetic field energy by changing the lattice spacing of the array. We also show that by moving the tip farther from or closer to the surface, which changes the potential well from the tip, that the configurations of vortices can be modified. From the experiments, we show that this technique can be used to both extract the strengths of the relative energy scales in this system and other superconducting systems, as well as for manipulating the vortex configurations for quantum computation applications.
We investigate the feasibility of using ultracold neutral atoms trapped near a thin superconductor to study vortex noise close to the Kosterlitz-Thouless-Berezinskii transition temperature. Alkali atoms such as rubidium probe the magnetic field produced by the vortices. We show that the relaxation time T1 of the Zeeman sublevel populations can be conveniently adjusted to provide long observation times. We also show that the transverse relaxation times T2 for Zeeman coherences are ideal for studying the vortex noise. We briefly consider the motion of atom clouds held close to the surface as a method for monitoring the vortex motion.
We describe a new method for observing magnetic periodicities, both spatial and temporal, ultimately capable of resolving wavelengths between 100 and 105 \AA{} and frequencies between 0.5 and 500 MHz. We have used this method to observe the periodic magnetic field above the surface of a type-II superconductor containing a lattice of flux lines which moves under transport currents. We believe that the measurements are the first direct observations of the flux-flow velocity in a type-II superconductor.
A lattice of Abrikosov vortices in type II superconductors is characterized by a periodic modulation of the magnetic induction perpendicular to the applied magnetic field. For a coherent vortex motion under the action of a transport current, the magnetic induction at a given point of the sample varies in time with a washboard frequency fWB = v/d, where v is the vortex velocity and d is the distance between the vortices in the direction of motion. Here, by using a spectrum analyzer connected to a 50 nm-wide Au nanowire meander near the surface of a superconducting Nb film, we detect an ac voltage induced by coherently moving fluxons. The voltage is peaked at the washboard frequency, fWB, and its subharmonics, fTOF = fWB/5, determined by the antenna width. By sweeping the dc current value, we reveal that fWB can be tuned from 100 MHz to 1.5 GHz, thereby demonstrating that patterned normal metal/superconductor nanostructures can be used as dc-tunable generators operating in the radiofrequency range.
In order to investigate the vortex lattice melting transition in detail Bi2Sr2CaCu2O8+ δ Scanning Hall probe microscopy (SHPM), a novel scanned probe magnetic imaging method, has been developed for single crystals. Using an integrated scanning tunnelling microscope (STM) or atomic force microscope (AFM) tip, the topography of a material can be recorded in real-time, enabling correlation with magnetic images later on. The numerous advantages of SHPM over other methods include its almost complete non-invasiveness and its suitability across a wide range of temperatures and magnetic fields. (0.3-300 Kelvin(K) and 0-7 Tesla (T), respectively), and its capacity to offer magnetic induction's z-component in quantitative maps. Photographing vortices at low temperatures in type II superconductors is made possible by the method's strong signal-to-noise ratio and reasonably good spatial resolution (>100 nm). Making use of models that GaAs/AlGaAs diameters between “0.4 µm - 0.8 µm high-resolution scanning Semiconductor 2DEG Hall probe microscopy (SHPM) Two orthogonal forms of flux structure, named Josephson vortices and pancake vortices, respectively, are generated in the crossing lattices domain of these highly anisotropic superconductors under tilted magnetic fields, according to research using Hall probe devices. (JVs and PVs)”. We see clear evidence for the formation about vortex lattice structures from H=+2.5 Oersted (Oe) gradually disappear.