Conventional superconductors disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. Consequently, ferromagnetic coupling between localized moments through the charge carriers is absent in s-wave superconductors. However, Rashba-type spin-orbit coupling (SOC) can profoundly alter the interaction between the charge carriers and localized spins. Here, we report long-range ferromagnetic order among the impurity moments on the surface of an s-wave SOC superconductor, Fe(Se, Te). We employ scanning superconducting quantum interference device microscopy and find suppressed vortex states but spontaneous magnetic domains in the superconducting state. Homogeneous remanent flux patterns are induced by applying supercurrents through the sample. The patterns are consistent with anomalous edge and bulk supercurrents generated by long-range in-plane magnetization related to the magneto-electric effect intrinsic to a Rashba superconductor. The anomalous supercurrents occur above a bias current threshold and follow hysteresis loops reminiscent of those of a ferromagnet. Surprisingly, the magnetic switching (super)current density is three orders of magnitude less than that in a ferromagnetic metal with SOC. These observations suggest an emergent ferromagnetic impurity state in a Rashba superconductor, which can be controlled by applying supercurrents.

The topological crystalline insulator SnTe exhibits surface-dependent Dirac cones, which are located at non-time-reversal-invariant momenta on the (001) and (110) surfaces, but at time-reversal-invariant momenta on the (111) surface. Motivated by the recent experimental evidence of Majorana vortex end modes (MVEMs) and their hybridization on the (001) surface [Liu et al., Nature (London) 633, 71 (2024)], we present a comprehensive investigation of Majorana vortex phases in SnTe with proximity-induced superconductivity, including topological classification, surface-state Hamiltonians analysis, and lattice model calculations. By utilizing rotational and magnetic mirror symmetries, we present two equivalent methods to reveal the topology of Majorana phases on different surfaces. We find that the MVEMs on the (001) and (110) surfaces are protected by both magnetic group and rotational symmetries. In contrast, the MVEMs on the (111) surface are protected by magnetic group or particle-hole symmetry. Due to the different properties of Dirac fermions in the P and M valleys on the (111) surfaces, including Fermi velocities and energy levels, we find that abundant vortex phase transitions can occur for the [111]-direction vortex. As the chemical potential shifts from the surface to bulk states, the number of robust MVEMs can change from 2 -* 1 -* 0. These vortex transitions are characterized by both Z winding number and Z2 pfaffian topological invariants.

The chiral edge current is the boundary manifestation of the Chern number of a quantum anomalous Hall (QAH) insulator. The van der Waals antiferromagnet MnBi2Te4 is theorized to be a QAH in odd-layers but has shown Hall resistivity below the quantization value at zero magnetic field. Here, we perform scanning superconducting quantum interference device (sSQUID) microscopy on these seemingly failed QAH insulators to image their current distribution. When gated to the charge neutral point, our device exhibits edge current, which flows unidirectionally on the odd-layer boundary both with vacuum and with the even-layers. The edge current chirality reverses with the magnetization of the bulk. Surprisingly, we find the edge channels coexist with finite bulk conduction even though the bulk chemical potential is in the band gap, suggesting their robustness under significant edge-bulk scattering. Our result establishes the existence of chiral edge currents in a topological antiferromagnet and offers an alternative for identifying QAH states.

The chiral edge current is the boundary manifestation of the Chern number of a quantum anomalous Hall (QAH) insulator. Its direct observation is assumed to require well-quantized Hall conductance, and is so far lacking. The recently discovered van der Waals antiferromagnet MnBi$_2$Te$_4$ is theorized as a QAH in odd-layers but has shown Hall resistivity below the quantization value at zero magnetic field. Here, we perform scanning superconducting quantum interference device (sSQUID) microscopy on these seemingly failed QAH insulators to image their current distribution. When gated to the charge neutral point, our device exhibits edge current, which flows unidirectionally on the odd-layer boundary both with vacuum and with the even-layer. The chirality of such edge current reverses with the magnetization of the bulk. Surprisingly, we find the edge channels coexist with finite bulk conduction even though the bulk chemical potential is in the band gap, suggesting their robustness under significant edge-bulk scattering. Our result establishes the existence of chiral edge currents in a topological antiferromagnet and offers an alternative for identifying QAH states.

Conventional superconductors naturally disfavor ferromagnetism because the supercurrent-carrying electrons are paired into anti-parallel spin singlets. In superconductors with strong Rashba spin-orbit coupling, impurity magnetic moments induce supercurrents through the spin-galvanic effect. As a result, long-range ferromagnetic interaction among the impurity moments may be mediated through such anomalous supercurrents in a similar fashion as in itinerant ferromagnets. Fe(Se,Te) is such a superconductor with topological surface bands, previously shown to exhibit quantum anomalous vortices around impurity spins. Here, we take advantage of the flux sensitivity of scanning superconducting quantum interference devices to investigate superconducting Fe(Se,Te) in the regime where supercurrents around impurities overlap. We find homogeneous remanent flux patterns after applying a supercurrent through the sample. The patterns are consistent with anomalous edge and bulk supercurrents generated by in-plane magnetization, which occur above a current threshold and follow hysteresis loops reminiscent of those of a ferromagnet. Similar long-range magnetic orders can be generated by Meissner current under a small out-of-plane magnetic field. The magnetization weakens with increasing temperature and disappears after thermal cycling to above superconducting critical temperature; further suggesting superconductivity is central to establishing and maintaining the magnetic order. These observations demonstrate surface anomalous supercurrents as a mediator for ferromagnetism in a spin-orbit coupled superconductor, which may potentially be utilized for low-power cryogenic memory.

Monolayers of a prototypical cuprate high transition-temperature (T C ) superconductor Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) was recently found to show T C and other electronic properties similar to those of the bulk. The robustness of superconductivity in an ideal two-dimensional (2D) system was an intriguing fact that defied the Mermin-Wagner theorem. Here, we took advantage of the high sensitivity of scanning SQUID susceptometry to image the phase stiffness throughout the phase transition of Bi2212 in the 2D limit. We found susceptibility oscillated with flux between diamagnetism and paramagnetism in a Fraunhofer-like pattern up till T C . The temperature and sample size-dependence of the modulation period agreed well with our Coulomb gas analogy of a finite 2D system based on Berezinskii–Kosterlitz–Thouless (BKT) transition. In the multilayers, the susceptibility oscillation differed in a small temperature regime below T C in consistent with a dimensional-crossover led by interlayer coupling. Serving as strong evidence of BKT transition in the bulk, there appeared a sharp superfluid density jump at zero-field and paramagnetism at small fields just below T C . These results unified the phase transitions from the monolayer Bi2212 to the bulk as BKT transition with finite interlayer coupling. This elucidating picture favored the pre-formed pairs scenario for the underdoped cuprates regardless of lattice dimensionality.

Vortices are topological defects of type-II superconductors in an external magnetic field. In a similar fashion to a quantum anomalous Hall insulator, quantum anomalous vortex (QAV) spontaneously nucleates due to orbital-and-spin exchange interaction between vortex core states and magnetic impurity moment, breaking time-reversal symmetry (TRS) of the vortex without an external field. Here, we used scanning superconducting quantum interference device microscopy (sSQUID) to search for its signatures in iron-chalcogenide superconductor Fe(Se,Te). Under zero magnetic field, we found a stochastic distribution of isolated anomalous vortices and antivortices with flux quanta $Φ_0$. By applying a small local magnetic field under the coil of the nano-SQUID device, we observed hysteretic flipping of the vortices reminiscent of the switching of ferromagnetic domains, suggesting locally broken-TRS. We further showed vectorial rotation of a flux line linking a paired vortex-antivortex with the local field. These unique properties of the anomalous vortices satisfied the defining criteria of QAV. Our observation suggests a quantum vortex phase with spontaneous broken-TRS in a high-temperature superconductor.

The relationship between charge-density-wave (CDW) and superconductivity (SC), two vital physical phases in condensed matter physics, has always been the focus of scientists' research over the past decades. Motivated by this research hotspot, we systematically studied the physical properties of the layered telluride chalcogenide superconductors CuIr2-xAlxTe4. Through the resistance and magnetization measurements, we found that the CDW order was destroyed by a small amount of Al doping. Meanwhile, the superconducting transition temperature (Tc) kept changing with the change of doping amount and rose towards the maximum value of 2.75 K when x = 0.075. The value of normalized specific heat jump for the highest Tc sample CuIr1.925Al0.075Te4 was 1.53, which was larger than the BCS value of 1.43 and showed that bulk superconducting nature. In order to clearly show the relationship between SC and CDW states, we propose a phase diagram of Tc vs. doping content.

Scanning superconducting quantum interference device microscopy (sSQUID) is currently one of the most effective methods for direct and sensitive magnetic flux imaging on the mesoscopic scale. A SQUID-on-chip design allows integration of field coils for susceptometry in a gradiometer setup which is very desirable for measuring magnetic responses of quantum matter. However, the spatial resolution of such a design has largely been limited to micrometers due to the difficulty in approaching the sample. Here, we used electron beam lithography technology in the fabrication of the 3D nano-bridge-based SQUID devices to prepare pick-up coils with diameters down to 150 nm. Furthermore, we integrated the deep silicon etching process in order to minimize the distance between the pick-up coil and the wafer edge. Combined with a tuning-fork-based scanning head, the sharpness of the etched chip edge enables a precision of 5 nm in height control. By scanning measurements on niobium chessboard samples using these improved SQUID devices, we demonstrate sub-micron spatial resolutions in both magnetometry and susceptometry, significantly better than our previous generations of nano-SQUIDs. Such improvement in spatial resolution of SQUID-on-chip is a valuable progress for magnetic imaging of quantum materials and devices in various modes.

Here, we report the effect of electron doping Zn for Cu on the physical properties of Cu0.5IrTe2. Slight Zn doping concentration (x) becomes detrimental to the charge density wave (CDW) order, while the superconducting state can persist over a large change in chemical composition. The x dependence of the superconducting transition temperature (T-c) exhibits a weak dome-like shape with the highest T-c of 2.82 K at x = 0.5, whereas there is only a subtle change in T-c. The normalized electronic specific heat Delta C-el./gamma T-c value of 1.45 for the optimal doping composition Zn0.25Cu0.25IrTe2 approaches the Bardeen-Cooper-Schrieffer (BCS) value (1.43), indicating the bulk nature of superconductivity. Magnetization and resistivity results further imply that our Zn-doped Cu0.5IrTe2 are type II superconductors. We propose that these robust (ZnxCu1-x)(0.5)IrTe2 (0 <= x <= 0.9) superconductors may be suited to some research based on exfoliated crystal flakes and film devices.

Here, we report the effect of electron doping Zn for Cu on the physical properties of Cu₀.₅IrTe₂. Slight Zn doping concentration (x) becomes detrimental to the charge density wave (CDW) order, while the superconducting state can persist over a large change in chemical composition. The x dependence of the superconducting transition temperature (Tc) exhibits a weak dome-like shape with the highest Tc of 2.82 K at x = 0.5, whereas there is only a subtle change in Tc. The normalized electronic specific heat ΔCₑₗ./γTc value of 1.45 for the optimal doping composition Zn₀.₂₅Cu₀.₂₅IrTe₂ approaches the Bardeen–Cooper–Schrieffer (BCS) value (1.43), indicating the bulk nature of superconductivity. Magnetization and resistivity results further imply that our Zn-doped Cu₀.₅IrTe₂ are type II superconductors. We propose that these robust (ZnₓCu₁–ₓ)₀.₅IrTe₂ (0 ≤ x ≤ 0.9) superconductors may be suited to some research based on exfoliated crystal flakes and film devices.

Transition metal dichalcogenides (TMDCs) usually exhibit layered polytypic structures due to the weak interlayer coupling. 2H-NbSe2 is one of the most widely studied in the pristine TMDC family due to its high superconducting transition temperature (Tc = 7.3K) and the occurrence of a charge-density wave (CDW) order below 33 K. The coexistence of CDW with superconductivity poses an intriguing open question about the relationship between Fermi surface nesting and Cooper pairing. Past studies of this issue have mostly been focused on doping 2H-NbSe2 by 3d transition metals without significantly changing its crystal structure. Here we replaced the Se by Te in 2H-NbSe2 in order to design a new 1T polytype layered TMDC NbSeTe, which adopts a trigonal structure with space group P-3m1. We successfully grew large size and high-quality single crystals of 1T-NbSeTe via the vapor transport method using I2 as the transport agent. Temperature-dependent resistivity and specific heat data revealed a bulk Tc at 1.3 K, which is the first observation of superconductivity in pure 1T-NbSeTe phase. This compound enlarged the family of superconducting TMDCs and provides an opportunity to study the interplay between CDW and superconductivity in the trigonal structure.

The interplay between superconductivity and charge density wave (CDW)/metal-to-insulator transition (MIT) has long been interested and studied in condensed matter physics. Here we study systematically the charge density wave and superconductivity properties in the solid solutions Zn1-xCuxIr2-yN(N = Al, Ti, Rh)yTe4. Resistivity, magnetic susceptibility and specific heat measurements indicate that the CDW state was suppressed immediately while the superconducting critical temperature (Tc) differs from each system. In the Al- and Ti-substitution cases, Tc increase as y increases and reaches a maximum around 2.75 K and 2.84 K respectively at y = 0.075, followed by a decrease of Tc before the chemical phase boundary is reached at y = 0.2. Nevertheless, Tc decreases monotonously with Rh-doping content y increases and disappears above 0.3 with measuring temperature down to 2 K. Surprisingly, in the Zn1-xCuxIr2Te4 solid solution, Tc enhances as x increases and reaches a maximum value of 2.82 K for x = 0.5 but subsequently survives over the whole doping range of 0.00 - 0.9 despite Tc changes slightly with higher doping content, which differs from the observation of zinc doping suppressing the superconductivity quickly in the high Tc cuprate superconductors. The specific heat anomaly at the superconducting transitions for the representative optimal doping samples are all slightly higher than the BCS value of 1.43 and indicate bulk superconductivity in these compounds. Finally, the CDW transition temperature (TCDW) and superconducting transition temperature (Tc) vs. x/y content phase diagrams of Zn1-xCuxIr2-yN(N = Al, Ti, Rh)yTe4 have been established and compared, which offers good opportunity to study the competition between CDW and superconductivity in the telluride chalcogenides.

The iron-chalcogenide high temperature superconductor Fe(Se,Te) (FST) has been reported to exhibit complex magnetic ordering and nontrivial band topology which may lead to novel superconducting phenomena. However, the recent studies have so far been largely concentrated on its band and spin structures while its mesoscopic electronic and magnetic response, crucial for future device applications, has not been explored experimentally. Here, we used scanning superconducting quantum interference device microscopy for its sensitivity to both local diamagnetic susceptibility and current distribution in order to image the superfluid density and supercurrent in FST. We found that in FST with 10% interstitial Fe, whose magnetic structure was heavily disrupted, bulk superconductivity was significantly suppressed whereas edge still preserved strong superconducting diamagnetism. The edge dominantly carried supercurrent despite of a very long magnetic penetration depth. The temperature dependences of the superfluid density and supercurrent distribution were distinctively different between the edge and the bulk. Our Heisenberg modeling showed that magnetic dopants stabilize anti-ferromagnetic spin correlation along the edge, which may contribute towards its robust superconductivity. Our observations hold implication for FST as potential platforms for topological quantum computation and superconducting spintronics.

We develop superconducting quantum interference device (SQUID) probes based on 3D nano-bridge junctions for the scanning SQUID microscopy. The use of these nano-bridge junctions enables imaging in the presence of a high magnetic field. Conventionally, a superconducting ground layer has been employed for better magnetic shielding. In our study, we prepare a number of scanning SQUID probes with and without a ground layer to evaluate their performance in external magnetic fields. The devices show the improved magnetic modulation up to 1.4 T. It is found that the ground layer reduces the inductance, and increases the modulation depth and symmetricity of the gradiometer design in the absence of the field. However, the layer is not compatible with the use of the scanning SQUID probe in the field because it decreases its working field range. Moreover, by adding the layer, the mutual inductance between the feedback coil and the SQUID also decreases linearly as a function of the field.

We report systematic magnetotransport measurements and ab initio calculations on single-crystalline TaP2, a member of the transition-metal dipnictides. We observed unsaturated magnetoresistance (MR) reaching similar to 700% at a magnetic field (B) of 9 T at 2 K when B is perpendicular to the electric current (I). We also found negative longitudinal MR (n-LMR) when B slightly deviates from the direction of I along with striking Shubnikov-de Haas (SdH) oscillations. Our analysis on the SdH oscillations uncovers three fundamental magnetic oscillation frequencies of 72, 237, and 356 T, slightly different from the theoretical calculations which reveal one hole pocket and two electron pockets at the L point and one electron pocket at the Z point of the Brillouin zone. The inconsistency might be due to that one Fermi pocket is out of the detection capability by our measurements. The analysis also reveals a nonzero Berry phase, indicating nontrivial band topology. The n-LMR could be fitted with the Adler-Bell-Jackiw chiral anomaly equation but the origin remains yet ambiguous. The ab initio calculations suggest TaP2 as a weak topological insulator with the Z(2) indices of (0; 111), which exhibits topological surface states on the (001) surface.

FeTe, a non-superconducting parent compound in the iron-chalcogenide family, becomes superconducting after annealing in oxygen. Under the presence of magnetism, spin-orbit coupling, inhomogeneity and lattice distortion, the nature of its superconductivity is not well understood. Here we combine the mutual inductance technique with magneto transport to study the magnetization and superconductivity of FeTe thin films. It is found that the films with the highest T-C show non-saturating superfluid density and a strong magnetic hysteresis distinct from that in a homogeneous superconductor. Such a hysteresis can be well explained by a two-level critical state model and suggests the importance of granularity to superconductivity in this compound.

Here we report the effect of structural and superconductivity properties on Ru-doped $\mathrm{CuI}{\mathrm{r}}_{2}\mathrm{T}{\mathrm{e}}_{4}$ telluride chalcogenide. X-ray diffraction results suggest that $\mathrm{CuI}{\mathrm{r}}_{2\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x}\mathrm{T}{\mathrm{e}}_{4}$ maintains the disordered trigonal structure with space group $P\overline{3}m1$ (no. 164) for $x$ \ensuremath{\le} 0.3. The lattice constants, $a$ and $c$, both decrease with increasing Ru content. Temperature-dependent resistivity, magnetic susceptibility, and specific heat measurements are performed to characterize the superconducting properties systematically. Our results suggest that the optimal doping level for superconductivity in $\mathrm{CuI}{\mathrm{r}}_{2\ensuremath{-}x}\mathrm{R}{\mathrm{u}}_{x}\mathrm{T}{\mathrm{e}}_{4}$ is $x=0.05$, where ${T}_{c}$ is 2.79 K with the Sommerfeld constant \ensuremath{\gamma} of $11.52\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$, and the specific heat anomaly at the superconducting transition, $\mathrm{\ensuremath{\Delta}}C/\ensuremath{\gamma}{T}_{c}$, is approximately 1.51, which is slightly higher than the Bardeen-Cooper-Schrieffer value of 1.43 and demonstrates bulk superconductivity in our $\mathrm{CuI}{\mathrm{r}}_{1.95}\mathrm{R}{\mathrm{u}}_{0.05}\mathrm{T}{\mathrm{e}}_{4}$ compound. The values of the lower ${{H}_{c1}(0)}$ and upper ${{H}_{c2}(0)}$ critical field calculated from isothermal magnetization {M(H)} and magnetotransport ${\ensuremath{\rho}(T,H)}$ measurements are 0.98 and 2.47 kOe, respectively, signifying that the compound is clearly a type-II superconductor. Finally, a ``domelike'' shape superconducting transition temperature $({T}_{c})$ vs $x$ content phase diagram has been established. A low substitution ($x=0.03$) of Ru for Ir leads to the disappearance of the charge density wave transition, while ${T}_{c}$ rises and reaches a maximum value of 2.79 K at $x=0.05$, followed by a decrease of ${T}_{c}$ as $x$ increases. This feature of the competition between the charge density wave we have established and the superconductivity could be caused by tuning the Fermi surface and density of states with Ru chemical doping.

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 μФ0/√Hz at 100 Hz.We demonstrate scanning magnetometry, susceptometry and current magnetometry on some test samples using these nano-SQUIDs.

2H-NbSe2 is one of the most widely researched transition metal dichalcogenide (TMD) superconductors, which undergoes charge-density wave (CDW) transition at TCDW about 33 K and superconducting transition at Tc of 7.3 K. To explore the relation between its superconductivity and Fermi surface nesting, we combined S substitution with Cu intercalation in 2H-NbSe2 to make CuxNbSe2-ySy. Upon systematic substitution of S and intercalation of Cu ions into 2H-NbSe2, we found that when the Cu and S contents increases, the Tc decreases in CuxNbSe2-ySy. While at higher x and y values, Tc keeps a constant value near 2 K, which is not commonly observed for a layered TMD. For comparison, we found the simultaneous substitution of Nb by Cu and Se by S in CuxNb1-xSe2-ySy lowered the Tc substantially faster. We construct a superconducting phase diagrams for our double-doping compounds in contrast with the related single-ions doping systems.