Taichiro Nishio, Vu Hung Dao, Qinghua Chen, Liviu F. Chibotaru, Kazuo Kadowaki, Victor V. Moshchalkov
In type-1.5 superconductors, vortices emerge in clusters, which grow in size with increasing magnetic field. These novel vortex clusters and their field dependence are directly visualized by scanning SQUID microscopy at very low vortex densities in MgB2 single crystals. Our observations are elucidated by simulations based on a two-gap Ginzburg-Landau theory in the type-1.5 regime.
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In this paper we discuss design concepts for increasing the spatial resolution, improving the sensitivity, and reducing the invasiveness in scanning Superconducting Quantum Interference Device (SQUID) microscope sensors with integrated flux pickup loops. This can be done not only by reducing the ground-rule line widths and spacings, but also by taking advantage of planarization, reducing flux noise through reducing the SQUID inductance, and reducing back-action through dispersive readouts or on-chip filtering.
We use a scanning superconducting quantum interference device (SQUID) to image the magnetic flux produced by a superconducting device designed for quantum computing. The nanometer-scale SQUID-on-tip probe reveals the flow of superconducting current through the circuit as well as the locations of trapped magnetic flux. In particular, maps of current flowing out of a flux-control line in the vicinity of a qubit show how these elements are coupled, providing insight on how to optimize qubit control.
Scanning SQUID susceptometry images the local magnetization and susceptibility of a sample. By accurately modeling the SQUID signal we can determine the physical properties such as the penetration depth and permeability of superconducting samples. We calculate the scanning SQUID susceptometry signal for a superconducting slab of arbitrary thickness with isotropic London penetration depth, on a non-superconducting substrate, where both slab and substrate can have a paramagnetic response that is linear in the applied field. We derive analytical approximations to our general expression in a numbe…
Scanning Superconducting QUantum Interference Device (SQUID) microscopy is a powerful tool for imaging local magnetic properties of materials and devices, but it requires a low-vibration cryogenic environment, traditionally achieved by thermal contact with a bath of liquid helium or the mixing chamber of a "wet" dilution refrigerator. We mount a SQUID microscope on the 3 K plate of a Bluefors cryocooler and characterize its vibration spectrum by measuring SQUID noise in a region of sharp flux gradient. By implementing passive vibration isolation, we reduce relative sensor-sample vibrations to …
We developed a scanning DC SQUID microscope with novel readout electronics capable of wideband sensing RF magnetic fields from 50 to 200 MHz and simultaneously providing closed-loop response at kHz frequencies. To overcome the 20 MHz bandwidth limitation of traditional closed-loop SQUIDs, a flux-modulated closed loop simultaneously locks the SQUID quasi-static flux and flux-biases the SQUID for amplification of the RF flux up to $Φ_0/4$ in amplitude. Demodulating the SQUID voltage with a double lock-in technique yields a signal representative of both the amplitude and phase of the RF flux. Thi…
Superconducting QUantum Interference Device (SQUID) microscopy has excellent magnetic field sensitivity, but suffers from modest spatial resolution when compared with other scanning probes. This spatial resolution is determined by both the size of the field sensitive area and the spacing between this area and the sample surface. In this paper we describe scanning SQUID susceptometers that achieve sub-micron spatial resolution while retaining a white noise floor flux sensitivity of $\approx 2μΦ_0/Hz^{1/2}$. This high spatial resolution is accomplished by deep sub-micron feature sizes, well shie…
We investigate the magnetic field distribution near the surface of a magnetic superconductor when a magnetic source is placed close to the superconductor. The magnetic field distribution can be measured by magnetic force microscopy and scanning SQUID microscopy, from which one can extract both the penetration depth $λ_L$ and magnetic susceptibility $χ$. When the magnetic moments are parallel to the surface, one extracts $λ_L/\sqrt{1-4πχ}$. When the moments are perpendicular to the surface, one obtains $λ_L$. By changing the orientation of the crystal, one thus is able to extract both $χ$ and $…
We measure changes in the penetration depth $λ$ of the $T_c \approx 6$ K superconductor LaFePO. In the process scanning SQUID susceptometry is demonstrated as a technique for accurately measuring {\it local} temperature-dependent changes in $λ$, making it ideal for studying early or difficult-to-grow materials. $λ$ of LaFePO is found to vary linearly with temperature from 0.36 to $\sim$2 K, with a slope of 143$\pm$15 Å/K, suggesting line nodes in the superconducting order parameter. The linear dependence up to $\sim T_c/3$ is similar to the cuprate superconductors, indicating well-developed no…
Correlating DC SQUID Performance with the Location of Trapped Magnetic Flux Using Scanning SQUID Microscopy
本研究利用扫描超导量子干涉显微镜直接关联直流超导量子干涉器件性能与捕获磁通位置，发现无论护城河配置如何均存在优先钉扎位点，为电路布局与故障分析提供反馈。
We report real-space evidence for an in-plane vortex lattice (VL) transition in the multiband superconductor 2H-NbSe2, observed by laterally imaging subsurface vortices with scanning tunneling spectroscopy. With magnetic field applied || ab-surface up to 6.5 T at 300 mK, spatial maps of the zero-bias conductance revealed a stripe pattern whose dominant periodicity showed a discrete shift near 0.7 T. The stripes can be interpreted as the surface projection of a distorted hexagonal VL that undergoes a first-order reorientation transition. This transition occurs for field || [100] but not [110], …
Identifying the symmetry of the superconducting order parameter in the recently-discovered ferro-oxypnictide family of superconductors, RFeAsO$_{1-x}$F$_{y}$, where $R$ is a rare earth, is a high priority. Many of the proposed order parameters have internal $π$ phase shifts, like the d-wave order found in the cuprates, which would result in direction-dependent phase shifts in tunnelling. In dense polycrystalline samples, these phase shifts in turn would result in spontaneous orbital currents and magnetization in the superconducting state. We perform scanning SQUID microscopy on a dense polycry…
The Dirac semimetal PdTe${}_2$ becomes superconducting at a temperature $T_{c}=1.6$~K. Thermodynamic and muon spin rotation experiments support type-I superconductivity, which is unusual for a binary compound. A key property of a type-I superconductor is the intermediate state which presents a coexistence of superconducting and normal domains (flux structures) at magnetic fields lower than the thermodynamic critical field $H_{c}$. By means of scanning SQUID microscopy (SSM) we observe flux structures in the superconducting state of PdTe${}_2$. The flux structures are strongly history dependent…
The glass-like vortex distribution in pulsed laser deposited YBCO thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikel…
One of the critical milestones in the intensive pursuit of quantitative nanoscale magnetic imaging tools is achieving the level of sensitivity required for detecting the field generated by the spin magnetic moment μB of a single electron. Superconducting quantum interference devices (SQUIDs), which were traditionally the most sensitive magnetometers, could not hitherto reach this goal because of their relatively large effective size (of the order of 1 μm). Here we report self-aligned fabrication of nano-SQUIDs with diameters as small as 46 nm and with an extremely low flux noise of 50 nΦ0/Hz^1…
We designed and fabricated a new type of superconducting quantum interference device (SQUID) susceptometers for magnetic imaging of quantum materials. The 2-junction SQUID sensors employ 3D Nb nano-bridges fabricated using electron beam lithography. The two counter-wound balanced pickup loops of the SQUID enable gradiometric measurement and they are surrounded by a one-turn field coil for susceptibility measurements. The smallest pickup loop of the SQUIDs were 1 $μm$ in diameter and the flux noise was around 1 $μΦ_0/\sqrt{Hz}$ at 100 Hz. We demonstrate scanning magnetometry, susceptometry and …
Advanced SQUID-on-lever scanning probe for high-sensitivity magnetic microscopy with sub-100-nm spatial resolution
本研究通过晶圆级光刻与FIB铣削结合，在硅悬臂顶端制造出内环尺寸小至10nm的纳米铌SQUIDs，实现亚100nm分辨率、0.3μΦ₀/√Hz磁通量灵敏度及宽磁场工作范围，扩展了扫描SQUID显微镜的应用场景并展示了斯格明子等成像能力。
Recently, advances in film synthesis methods have enabled a study of extremely overdoped $La_{2-x}Sr_{x}CuO_{4}$. This has revealed a surprising behavior of the superfluid density as a function of doping and temperature, the explanation of which is vividly debated. One popular class of models posits electronic phase separation, where the superconducting phase fraction decreases with doping, while some competing phase (e.g. ferromagnetic) progressively takes over. A problem with this scenario is that all the way up to the dome edge the superconducting transition remains sharp, according to mutu…
Spatial resolution and point spread function of high-resolution scanning SQUID microscopy probes
研究高分辨率扫描 SQUID 显微镜探头的空间分辨率和点扩散函数，通过模拟磁通量耦合给出 SOL PSF 的数学描述，利用单个磁斯格明子测量真实 SOL 磁通量响应，证实计算出的 PSF 和空间分辨率。
The diamagnetic susceptibility of a superconductor is directly related to its superfluid density. Mutual inductance is a highly sensitive method for characterizing thin films; however, in traditional mutual inductance measurements, the measured response is a non-trivial average over the area of the mutual inductance coils, which are typically of millimeter size. Here we image localized, isolated features in the diamagnetic susceptibility of δ-doped SrTiO3, the 2-DES at the interface between LaAlO3 and SrTiO3, and Nb superconducting thin film systems using scanning superconducting quantum inter…
We present scanning SQUID microscopy data on the superconductors Sr2RuO4 (Tc = 1.5 K) and PrOs$_4$Sb$_{12}$ (Tc = 1.8 K). In both of these materials, superconductivity-related time-reversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However in neither Sr$_2$RuO$_4$ nor PrOs$_4$Sb$_{12}$ do we observe spontaneous superconductivity-related magnetization. In Sr$_2$RuO$_4$, many experimental results have been interpreted on the basis of a $px \pm ipy$ superconducting order parameter. This order parameter is expected to…
Scanning Superconducting Quantum Interference Device (SQUID) Susceptometry simultaneously images the local magnetic fields and susceptibilities above a sample with sub-micron spatial resolution. Further development of this technique requires a thorough understanding of the current, voltage, and flux characteristics of scanning SQUID susceptometers. These sensors often have striking anomalies in their current-voltage characteristics, which we believe to be due to electromagnetic resonances. The effect of these resonances on the performance of these SQUIDs is unknown. To explore the origin and i…
Constraints on a split superconducting transition under uniaxial strain in Sr$_2$RuO$_4$ from scanning SQUID microscopy
通过扫描SQUID显微镜与单轴应变实验，在Sr$_2$RuO$_4$中仅观察到单一超导转变，排除了双分量序参量导致的第二转变，为理论模型提供约束。
In the past, magnetic images acquired using scanning Superconducting Quantum Interference Device (SQUID) microscopy have been interpreted using simple models for the sensor point spread function. However, more complicated modeling is needed when the characteristic dimensions of the field sensitive areas in these sensors become comparable to the London penetration depth. In this paper we calculate the response of SQUIDs with deep sub-micron pickup loops to different sources of magnetic fields by solving coupled London's and Maxwell's equations using the full sensor geometry. Tests of these calc…
We have designed and characterized a micro-SQUID with dispersive readout for use in low temperature scanning probe microscopy systems. The design features a capacitively shunted RF SQUID with a tunable resonance frequency from 5 to 12 GHz, micrometer spatial resolution and integrated superconducting coils for local application of magnetic fields. The SQUID is operated as a nonlinear oscillator with a flux- and power-dependent resonance frequency. Measurements for device characterization and noise benchmarking were carried out at 4 K. The measured flux noise above 10 kHz at 4 K is $80\;\mathrm{…
Scanning superconducting quantum interference device (SQUID) microscopy is a magnetic imaging technique combining high-field sensitivity with nanometer-scale spatial resolution. State-of-the-art SQUID-on-tip probes are now playing an important role in mapping correlation phenomena, such as superconductivity and magnetism, which have recently been observed in two-dimensional van der Waals materials. Here, we demonstrate a scanning probe that combines the magnetic and thermal imaging provided by an on-tip SQUID with the tip-sample distance control and topographic contrast of a non-contact atomic…
Vortex dynamics induced by scanning SQUID susceptometry
本研究利用扫描SQUID磁强计观测铌薄膜临界温度附近由交流场诱导的涡旋-反涡旋对动力学导致的非线性耗散响应，通过建模构建涡旋运动实空间图像，拓展了该技术的应用范围并奠定相关微观研究基础。
Fabrication of Nb and MoGe SQUID-on-tip probes by magnetron sputtering
本文提出利用常规磁控溅射制备Nb和MoGe尖端SQUID探针，实现50-80纳米直径、低噪声及高场工作，简化工艺并扩展材料适用性。
Direct observation of vortices by the scanning SQUID microscopy was made on large mesoscopic disks of an amorphous MoGe thin film. Owing to the weak pinning nature of the amorphous film, vortices are able to form geometry induced, (quasi-)symmetric configurations of polygons and concentric shells in the large disks. Systematic measurements made on selected disks allow us to trace not only how the vortex pattern evolves with magnetic field, but also how the vortex polygons change in size and rotate with respect to the disk center. The results are in good agreement with theoretical consideration…
We describe a new type of scanning probe microscope based on a superconducting quantum interference device (SQUID) that resides on the apex of a sharp tip. The SQUID-on-tip is glued to a quartz tuning fork which allows scanning at a tip-sample separation of a few nm. The magnetic flux sensitivity of the SQUID is 1.8 μ_0/Hz^{1/2} and the spatial resolution is about 200 nm, which can be further improved. This combination of high sensitivity, spatial resolution, bandwidth, and the very close proximity to the sample provides a powerful tool for study of dynamic magnetic phenomena on the nanoscale.…
Superconducting Quantum Interference Devices (SQUIDs) can have excellent spin sensitivity depending on their magnetic flux noise, pick-up loop diameter, and distance from the sample. We report a family of scanning SQUID susceptometers with terraced tips that position the pick-up loops 300 nm from the sample. The 600 nm - 2 um pickup loops, defined by focused ion beam, are integrated into a 12-layer optical lithography process allowing flux-locked feedback, in situ background subtraction and optimized flux noise. These features enable a sensitivity of ~70 electron spins per root Hertz at 4K.
In this review I discuss the application of scanning magnetic imaging to fundamental studies of superconductors, concentrating on three scanning magnetic microscopies - scanning SQUID microscopy (SSM), scanning Hall bar microscopy (SHM), and magnetic force microscopy (MFM). I briefly discuss the history, sensitivity, spatial resolution, invasiveness, and potential future developments of each technique. I then discuss a selection of applications of these microscopies.
We present a novel method, based on vortex imaging by low-temperature scanning electron microscopy (LTSEM), to directly image the sheet-current distribution in YBa2Cu3O7 dc SQUID washers. We show that the LTSEM vortex signals are simply related to the scalar stream function describing the vortex-free circulating sheet-current distribution J. Unlike previous inversion methods that infer the current distribution from the measured magnetic field, our method uses pinned vortices as local detectors for J. Our experimental results are in very good agreement with numerical calculations of J.
We present a method to produce superconducting tips to be used in Scanning Tunneling Microscopy/Spectroscopy experiments. We use these tips to investigate the evolution of the electronic density of states of NbSe2 from 0.3K up to its critical temperature (7.2K). The use of a superconducting tip (Pb) as ounterelectrode provides an enhancement of the different features related to the DOS of NbSe2 in the tunneling conductance curves, along all the studied thermal range. The analysis of the experimental results gives evidence of the presence of multiband superconductivity in NbSe2.
We used low-temperature scanning electron microscopy (LTSEM) for imaging quantized magnetic flux (vortices) in direct current (dc) superconducting quantum interference devices (SQUIDs) with approximately 1 micron spatial resolution at temperature T=77 K in a controllable magnetic field up to 20 microTesla. We demonstrate that LTSEM allows to image the spatial distribution of vortices in a YBa2Cu3O7/SrTiO3/YBa2Cu3O7 multilayer thin-film structure consisting of a dc SQUID washer with an integrated input coil on top. Simultaneously, we are able to measure the low-frequency noise of the sample und…
Nanoscale superconducting quantum interference devices (SQUIDs) demonstrate record sensitivities to small magnetic moments, but are typically sensitive only to the field component that is normal to the plane of the SQUID and out-of-plane with respect to the scanned surface. We report on a nanoscale three-junction Pb SQUID which is fabricated on the apex of a sharp tip. Because of its three-dimensional structure, it exhibits a unique tunable sensitivity to both in-plane and out-of-plane fields. We analyze the two-dimensional interference pattern from both numerical and experimental points of vi…
Direct observation of quantum vortex fractionalization in multiband superconductors
该研究利用扫描隧道显微镜首次在多带超导体KFe2As2表面观测到量子涡旋核心分数化现象，证实整数磁通涡旋可分裂为分数涡旋，为研究分数统计准粒子提供了新平台。
We present very low temperature scanning tunneling microscopy (STM) experiments on single crystalline samples of the superconductor $β$-Bi$_2$Pd. We find a single fully isotropic superconducting gap. However, the magnetic field dependence of the intervortex density of states is higher than the one expected in a single gap superconductor, and the hexagonal vortex lattice is locked to the square atomic lattice. Such increase in the intervortex density of states and vortex lattice locking have been found in superconductors with multiple superconducting gaps and anisotropic Fermi surfaces. We comp…
Miniaturization of superconducting quantum interference devices (SQUIDs) is of major importance for the development of sensitive scanning nanoscale magnetometry tools. The high sensitivity of nanoSQUIDs is restricted, however, to only particular periodic values of the applied magnetic field, making accurate measurements at intermediate values of the field impossible. We present a theoretical investigation of a multi-terminal, multi-junction SQUID (mSQUID) that lifts this limitation by providing electrical means for a continuous shift of the quantum interference pattern with respect to the appl…
The superconductivity of a kagome superconductor CsV3Sb5 is studied by scanning tunneling microscopy / spectroscopy at an ultralow temperature with high resolution. Two kinds of superconducting gaps with multiple sets of coherent peaks and residual zero-energy density of states are observed on both half-Cs and Sb surfaces, implying multiband superconductivity with gap nodes. Sixfold star-shaped magnetic vortex is observed with conventional Caroli-de Gennes-Matricon bound states inside. Magnetic impurities suppress the superconductivity, while nonmagnetic impurities do not, suggesting the absen…
Scanning SQUID study of ferromagnetism and superconductivity in infinite-layer nickelates
本文利用扫描SQUID研究三种无限层镍酸盐薄膜，发现铁磁背景和非居里-外斯动态磁化率可能源于NiO$_x$纳米颗粒，并观测到不均匀抗磁屏蔽、T线性超流密度及超导涡旋，确定了Nd样品的Pearl长度。
The vortex electronic structure in the multiband superconductor NbSe2 is studied by means of Scanning Tunneling Spectroscopy (STS) using a superconducting tip. The use of a superconducting tip (Pb) as a probe provides an enhancement of the different features related to the DOS of NbSe2 in the tunneling conductance curves. This use allows the observation of rich patterns of electronic states in the conductance images around the vortex cores in a wide range of temperature, as well as the simultaneous acquisition of Josephson current images in the vortex state.
Direct Imaging of Transition-Edge Sensors with Scanning SQUID Microscopy
本文利用扫描SQUID显微镜对Al-Mn过渡边缘传感器进行磁成像，揭示Nb引线附近的纵向邻近效应及其对转变宽度和电阻温度依赖性的影响，为超导探测器局部表征与优化设计提供了新方法。
We use low-temperature scanning electron microscopy combined with SQUID detection of magnetic flux to image vortices and to investigate low-frequency flux noise in YBa$_2$Cu$_3$O$_7$ thin film SQUIDs. The low-frequency flux noise shows a nonlinear increase with magnetic cooling field up to 60 $μ$T. This effect is explained by the surface potential barrier at the SQUID hole. By correlating flux noise data with the spatial distribution of vortices, we obtain information on spatial fluctuations of vortices on a microscopic scale, e.g. an average vortex hopping length of approximately 10 nm.
Supercurrent Diode Effect in Josephson Interferometers with Multiband Superconductors
研究揭示了多带超导体在约瑟夫森干涉仪中通过磁场调控实现非 reciprocity 超流体整流的机制，展示了时间反演对称性破缺对整流特性的影响。
Scanning tunneling experiments on single crystals of superconducting FeTe$_{0.55}$Se$_{0.45}$ have recently provided evidence for discrete energy levels inside vortices. Although predicted long ago, such levels are seldom resolved due to extrinsic (temperature, instrumentation) and intrinsic (quasiparticle scattering) limitations. We study a microscopic multiband model with parameters appropriate for FeTe$_{0.55}$Se$_{0.45}$. We confirm the existence of well-separated bound states and show that the chemical disorder due to random occupation of the chalcogen site does not affect significantly t…
We propose a novel type of magnetic scanning probe sensor, based on a single planar Josephson junction with a magnetic barrier. The planar geometry together with high magnetic permeability of the barrier helps to focus flux in the junction and thus enhance the sensitivity of the sensor. As a result, it may outperform equally sized SQUID both in terms of the magnetic field sensitivity and the spatial resolution in one scanning direction. We fabricate and analyze experimentally sensor prototypes with a superparamagnetic CuNi and a ferromagnetic Ni barrier. We demonstrate that the planar geometry…
We present extensive Scanning Tunneling Spectroscopy (STM/S) measurements at low temperatures in the multiband superconductor MgB$_2$. We find a similar behavior in single crystalline samples and in single grains, which clearly shows the partial superconducting density of states of both the $π$ and $σ$ bands of this material. The superconducting gaps corresponding to both bands are not single valued. Instead, we find a distribution of superconducting gaps centered around 1.9mV and 7.5mV, corresponding respectively to each set of bands. Interband scattering effects, leading to a single gap stru…
Scanning nanoscale superconducting quantum interference devices (nanoSQUIDs) are of growing interest for highly sensitive quantitative imaging of magnetic, spintronic, and transport properties of low-dimensional systems. Utilizing specifically designed grooved quartz capillaries pulled into a sharp pipette, we have fabricated the smallest SQUID-on-tip (SOT) devices with effective diameters down to 39 nm. Integration of a resistive shunt in close proximity to the pipette apex combined with self-aligned deposition of In and Sn, have resulted in SOT with a flux noise of 42 n$Φ_0$Hz$^{-1/2}$, yiel…
We examine the collective behavior of two-dimensional nonlinear superconducting metamaterials using a non-contact spatially resolved imaging technique. The metamaterial is made up of sub-wavelength nonlinear oscillators in a strongly coupled 27x27 planar array of radio-frequency Superconducting QUantum Interference Devices (rf SQUIDs). By using low-temperature laser scanning microscopy we image microwave currents in the driven SQUIDs while in non-radiating dark modes and identify the clustering and uniformity of like-oscillating meta-atoms. We follow the rearrangement of coherent patterns due …
Leggett Modes in Dirac Semimetals
本文提出通过交流驱动SQUID明确检测Leggett模式及其频率，并据此在邻近Al诱导超导的狄拉克半金属Cd3As2中确认了双带超导态下Leggett模式的存在。
Band-Selective Tunneling and Anisotropic Multiband Superconductivity in V$_2$Ga$_5$
通过热力学和光谱学结合第一性原理计算，揭示了V$_2$Ga$_5$中带选择性隧穿导致方向依赖的能隙层级反转，建立了各向异性多带s波配对模型。
We analyze how the coherence peaks observed in Scanning Tunneling Spectroscopy (STS) of cuprate high temperature superconductors are transferred from the cuprate layer to the oxide layers adjacent to the STS microscope tip. For this purpose, we have carried out a realistic multiband calculation for the superconducting state of Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ (Bi2212) assuming a short range d-wave pairing interaction confined to the nearest-neighbor Cu $d_{x^2-y^2}$ orbitals. The resulting anomalous matrix elements of the Green's function allow us to monitor how pairing is then induced not only w…
We use scanning SQUID microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: they tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in t…
Determining the exact pairing symmetry of the superconducting order parameter in candidate unconventional superconductors remains an important challenge. Recently a new method, based on phase sensitive quasiparticle interference measurements, was developed to identify gap sign changes in isotropic multiband systems. Here we extend this approach to the single-band and multiband nodal $d$-wave superconducting cases relevant, respectively, for the cuprates and likely for the infinite-layer nickelate superconductors. Combining analytical and numerical calculations, we show that the antisymmetrized…
Tunable vortex bound states in multiband CsV3Sb5-derived kagome superconductors
本研究在Ta、Ti掺杂的CsV3Sb5衍生kagome超导体中观测到可调涡旋束缚态，揭示了不同掺杂下的涡旋态特征差异，为理解该体系多带超导性及相关零偏压电导峰起源提供新视角。
Tapping-mode SQUID-on-tip Microscopy with Proximity Josephson Junctions
本文提出结合 AFM 与邻近结 nanoSQUID 的轻敲模式 SQUID-on-tip，实现无激光、无外部辐射下电流、磁性、耗散和形貌的同步纳米成像。
Scanning nanoscale superconducting quantum interference devices (SQUIDs) are gaining interest as highly sensitive microscopic magnetic and thermal characterization tools of quantum and topological states of matter and devices. Here we introduce a novel technique of collimated differential-pressure magnetron sputtering for versatile self aligned fabrication of SQUID on tip (SOT) nanodevices, which cannot be produced by conventional sputtering methods due to their diffusive, rather than the required directional point-source, deposition. The new technique provides access to a broad range of super…
We study the superconducting density of states and vortex lattice of single crystals of CaKFe$_4$As$_4$ using a scanning tunneling microscope (STM). This material has a critical temperature of $T_c= 35\,$ K, which is one of the highest among stoichiometric iron based superconductors (FeBSC) and is comparable to $T_c$ found near optimal doping in other FeBSC. Using quasi-particle interference we identify the hole sheets around the zone center and find that two superconducting gaps open in these sheets. The scattering centers are small defects that can be localized in the surface topography and …
We present direct imaging of magnetic flux structures in the anisotropic, spin-triplet superconductor Sr$_{2}$RuO$_{4}$ using a scanning $μ$SQUID microscope. Individual quantized vortices were seen at low magnetic fields. Coalescing vortices forming flux domains were revealed at intermediate fields. Based on our observations we suggest that a mechanism intrinsic to the material stabilizes the flux domains against the repulsive vortex-vortex interaction. Topological defects like domain walls can provide this, implying proof for unconventional chiral superconductivity.
We discuss the Meissner response to a known field source of superconductors having inhomogeneities in their penetration depth. We simplify the general problem by assuming that the perturbations of the fields by the penetration depth inhomogeneities are small. We present expressions for inhomogeneities in several geometries, but concentrate for comparison with experiment on planar defects, perpendicular to the sample surfaces, with superfluid densities different from the rest of the samples. These calculations are relevant for magnetic microscopies, such as Scanning Superconducting Quantum Inte…
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