We report numerical simulations of large-amplitude oscillations of a trapped vortex line under a strong ac magnetic field $H(t)=H\sinωt$ parallel to the surface. The power dissipated by an oscillating vortex segment driven by the surface ac Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass and a nonlinear Larkin-Ovchinnikov (LO) viscous drag coefficient $η(v)$. We show that the LO decrease of $η(v)$ with the vortex velocity $v$ can radically change the field dependence of the surface resistance $R_i(H)$ caused by trapped vortices. At low frequencies $R_i(H) $ exhibits a conventional increases with $H$, but as $ω$ increases, the surface resistance becomes a nonmonotonic function of $H$ which decreases with $H$ at higher fields. The effects of frequency, pin spacing and the mean free path $l_i $ on the field dependence of $R_{i}(H) $ were calculated. It is shown that, as the surface gets dirtier and $l_i$ decreases, the anomalous drop of $ R_{i}(H) $ with $H$ shifts to lower fields which can be much smaller than the lower critical magnetic field. Our numerical simulations also show that the LO decrease of $η(v)$ with $v$ can cause a vortex bending instability at high field amplitudes and frequencies, giving rise to the formation of dynamic kinks along the vortex. Measurements of $R_i(H)$ caused by sparse vortices trapped perpendicular to the surface can offer opportunities to investigate an extreme nonlinear dynamics of vortices driven by strong current densities up to the depairing limit at low temperatures. The behavior of $R_i(H)$ which can be tuned by varying the rf frequency or concentration of nonmagnetic impurities is not masked by strong heating effects characteristic of dc or pulse transport measurements.
The magnetic response related to paramagnetic Meissner effect (PME) is studied in a high quality single crystal ZrB12 with non-monotonic vortex-vortex interactions. We observe the expulsion and penetration of magnetic flux in the form of vortex clusters with increasing temperature. A vortex phase diagram is constructed which shows that the PME can be explained by considering the interplay among the flux compression, the different temperature dependencies of the vortex-vortex and the vortex-pin interactions, and thermal fluctuations. Such a scenario is in good agreement with the results of the …
We show that on superconducting spherical nanoshells, the co-existence of the Meissner state with a variety of vortex patterns drives the phase transition to higher magnetic fields. The spherical geometry leads to a Magnus-Lorentz force pushing the nucleating vortices and antivortices towards the poles, overcoming local pinning centers, preventing vortex-antivortex recombination and leading to the appearance of a Meissner belt around the sphere's equator. In sufficiently small and thin spherical shells paramagnetic vortex states can be stable, enabling spatial separation of freely moving shell…
We present an exact mathematical description of the Meissner effect and of the vortex state in periodic thin-layer superconductor/insulator structures with an arbitrary number of identical junctions N-1 (N is the number of superconducting layers) in the presence of a static parallel external field H. Based on an analytical analysis of the coupled static sine-Gordon (SG) equations for the phase differences, we obtain a complete classification of all possible types of physical solutions. We prove that at H>0 these equations admit only Meissner solutions and topological ''vortex-plane'' solutions…
From magnetisation measurements we provide evidence that the ferromagnetic superconductor RuSr$_{2}$GdCu$_{2}$O$_{8}$ with \QTR{it}{T}$_{c}$=45 K and \QTR{it}{T}$_{M}$=137 K exhibits a sizeable diamagnetic signal at low temperature (\QTR{it}{T}<\QTR{it}{T}$^{\QTR{it}{ms}}$=30 K) and low magnetic field (\QTR{it}{H}$^{ext}$<30 Oe), corresponding to a bulk Meissner-phase. At intermediate temperatures, \QTR{it}{T}$^{\QTR{it}{ms}}$<\QTR{it}{T}<\QTR{it}{T}$_{c}$, a spontaneous vortex phase forms which is characterized by unique thermal hysteresis effects. We argue that a recent negative report [C.W.…
We present an exact mathematical description of Josephson vortices and of the Meissner effect in periodic thin-layer superconductor/insulator structures with an arbitrary number of identical junctions N-1 (N is the number of superconducting layers) in terms of localized solutions to a system of differential equations for phase differences. We establish a general criterion of the existence of localized solutions. We show that Meissner solutions are characterized by several Josephson lengths [N/2 lengths for even N, and (N-1)/2 lengths for odd N]. We derive an exact expression for the superheati…
Using the non-linear Ginzburg-Landau (GL) theory, we study the magnetic response of different shaped samples in the field-cooled regime (FC). For high external magnetic fluxes, the conventional diamagnetic response under cooling down can be followed by the paramagnetic Meissner effect (PME). A second-order transition from a giant vortex state to a multi-vortex state, with the same vorticity, occurs at the second critical field which leads to the suppression of PME.
We investigate the vortex phase diagram in untwinned YBaCuO single crystals with columnar defects. These randomly distributed defects, produced by heavy ion irradiation, are expected to induce a ``Bose Glass'' phase of localized vortices characterized by a vanishing resistance and a Meissner effect for magnetic fields transverse to the defect axis. We directly observe the transverse Meissner effect using an array of Hall probe magnetometers. As predicted, the Meissner state breaks down at temperatures Ts that decrease linearly with increasing transverse magnetic field. However, Ts falls well b…
We theoretically find that finite size Fulde-Ferrell (FF) superconductor (which is characterized by spatially nonuniform ground state $Ψ\sim \text{exp}(-i{\bf q}_{FF}{\bf r})$ and $|Ψ|(r)=const$ in the bulk case, where $Ψ$ is a superconducting order parameter) has paramagnetic Meissner, vortex and 'onion' ground states with $|Ψ|(r) \neq const$. These states are realized due to boundary effect when the lateral size of superconductor $L \sim 1/q_{FF}$. We argue, that predicted states could be observed in thin disk/square made of superconductor-ferromagnet-normal metal trilayer with $L \simeq 150…
We demonstrate existence of non-pairwise interaction forces between vortices in multicomponent and layered superconducting systems. That is, in contrast to most common models, the interactions in a group of such vortices is not a universal superposition of Coulomb or Yukawa forces. Next we consider the properties of vortex clusters in Semi-Meissner state of type-1.5 two-component superconductors. We show that under certain condition non-pairwise forces can contribute to formation of very complex vortex states in type-1.5 regimes.
Nowadays superconductors serve in numerous applications, from high-field magnets to ultra-sensitive detectors of radiation. Mesoscopic superconducting devices, i.e. those with nanoscale dimensions, are in a special position as they are easily driven out of equilibrium under typical operating conditions. The out-of-equilibrium superconductors are characterized by non-equilibrium quasiparticles. These extra excitations can compromise the performance of mesoscopic devices by introducing, e.g., leakage currents or decreased coherence times in quantum devices. By applying an external magnetic field…
Exact analytic solutions are presented for the magnetic moment and screening currents in the Meissner state of superconductor strips with rectangular cross section in a perpendicular magnetic field and/or with transport current. The extension to finite London penetration is achieved by an elegant numerical method which works also for disks. The surface current in the specimen corners diverges as l^(-1/3) where l is the distance from the corner. This enhancement reduces the barrier for vortex penetration and should increase the nonlinear Meissner effect in d-wave superconductors.
We investigate the flux penetration patterns and matching fields of a long cylindrical wire of circular cross section in the presence of an external magnetic field. For this study we write the London theory for a long cylinder both for the mixed and Meissner state, with boundary conditions appropriate for this geometry. Using the Monte Carlo Simulated Annealing method, the free energy of the mixed state is minimized with respect to the vortex position and we obtain the ground state of the vortex lattice for N=3 up to 18 vortices. The free energy of the Meissner and mixed state provides express…
We present 3D numerical simulation results of moving vortex lattices in presence of 1D correlated disorder at zero temperature. Our results with field tilting confirm the theoritical predictions of a moving Bose glass phase, characterized by transverse pinning and dynamical transverse Meissner effect, the moving flux lines being localized along the correlated disorder direction. Beyond a critical transverse field, vortex lines exhibit along all their length a "kink" structure resulting from an effective static "tin roof" pinning potential in the transverse direction.
We report ac magnetic susceptibility and dc magnetization measurements on the superconducting ferromagnet UCoGe (with superconducting and Curie temperatures of $T_{\rm SC} \sim 0.5$~K and $T_{\rm Curie} \sim 2.5$~K, respectively). In the normal, ferromagnetic state ($T_{\rm SC} < T < T_{\rm Curie}$), the magnetization curve exhibits a hysteresis loop similar to that of a regular itinerant ferromagnet. Upon lowering the temperature below $T_{\rm SC}$, the spontaneous magnetization is unchanged, but the hysteresis is markedly enhanced. Even deeply inside the superconducting state, ferromagnetism…
An increase of the magnetic moment in superconductor/ferromagnet (S/F) bilayers V(40nm)/F [F$=$Fe(1,3nm), Co(3nm), Ni(3nm)] was observed using SQUID magnetometry upon cooling below the superconducting transition temperature Tc in magnetic fields of 10 Oe to 50 Oe applied parallel to the sample surface. A similar increase, often called the paramagnetic Meissner effect (PME), was observed before in various superconductors and superconductor/ferromagnet systems. To explain the PME effect in the presented S/F bilayers a model based on a row of vortices located at the S/F interface is proposed. Acc…
The paramagnetic Meissner effect (PME) is observed in small superconducting samples, and a number of controversial explanations of this effect are proposed, but there is as yet no clear understanding of its nature. In the present paper PME is considered on the base of the Ginzburg-Landau theory (GL). The one-dimensional solutions are obtained in a model case of a long superconducting cylinder for different cylinder radii R, the GL-parameters κand vorticities m. Acording to GL-theory, PME is caused by the presence of vortices inside the sample. The superconducting current flows around the vorte…
Traditionally, superconductors are categorized as type-I or type-II. Type-I superconductors support only Meissner and normal states, while type-II superconductors form magnetic vortices in sufficiently strong applied magnetic fields. Recently there has been much interest in superconducting systems with several species of condensates, in fields ranging from Condensed Matter to High Energy Physics. Here we show that the type-I/type-II classification is insufficient for such multicomponent superconductors. We obtain solutions representing thermodynamically stable vortices with properties falling …
A variational calculation for vortex penetration is presented. Variational trial functions for the Meissner state are combined with variational functions for a vortex near the surface. The latter is based on Clem's trial solutions for a vortex in bulk, which were adapted to include surface effects through consideration of an image vortex. Three variational parameters are considered, corresponding to the effective coherence length of the vortex, the effective penetration length for the Meissner currents, and the value of the order parameter at the surface. The results show that the last two var…
We investigated the nonlinear Meissner effect (NLME) in Nb$_3$Sn thin film coplanar resonators by measuring the resonance frequency as a function of a parallel magnetic field at different temperatures. We used low rf power probing in films thinner than the London penetration depth $λ(B)$ to significantly increase the field onset of vortex penetration and measure the NLME under equilibrium conditions. Contrary to the conventional quadratic increase of $λ(B)$ with $B$ expected in s-wave superconductors, we observed a nearly linear increase of the penetration depth with $B$. We concluded that thi…
The magnetic vortices in superconductors usually repel each other. Several cases are discussed when the vortex interaction has an attractive tail and thus a minimum, leading to vortex clusters and chains. Decoration pictures then typically look like in the intermediate state of type-I superconductors, showing lamellae or islands of Meissner state or surrounded by Meissner state, but with the normal regions filled with Abrikosov vortices that are typical for type-II superconductors in the mixed state. Such intermediate-mixed state was observed and investigated in detail in pure Nb, TaN and othe…
The electric charge density in mesoscopic superconductors with circular symmetry, i.e. disks and cylinders, is studied within the phenomenological Ginzburg-Landau approach. We found that even in the Meissner state there is a charge redistribution in the sample which makes the sample edge become negatively charged. In the vortex state there is a competition between this Meissner charge and the vortex charge which may change the polarity of the charge at the sample edge with increasing magnetic field. It is shown analytically that in spite of the charge redistribution the mesoscopic sample as a …
We have explored the vortex phase diagram in a spherical single crystal of niobium (T ~ 9.3 K) via isothermal and temperature dependent dc magnetization and ac-susceptibility measurements. The crystal has extremely weak pinning that can be inferred from the reversibility of loops below T_c. However, one can visualize the peak effect (PE) feature in the isothermal M-H loops up to T = 8 K. The PE is also prominent in isothermal ac-susceptibility data for H > 750 Oe. An interesting observation in the present study is the prominent fingerprints of surface superconductivity, starting just above the…
In a magnetic field, superconductivity is manifested by total magnetic field expulsion (Meissner effect) or by the penetration of integer multiples of the flux quantum Φ_0. Here we present experimental results revealing magnetic dipoles formed by Meissner current flowing around artificially introduced topological defects (lattice of antidots). By using scanning Hall probe microscopy, we have detected ordered magnetic dipole lattice generated at spatially periodic antidots in a Pb superconducting film. While the conventional homogeneous Meissner state breaks down, the total magnetic flux of the…
A spontaneous vortex state (SVS) between 30 K and 56 K was observed for the weak-ferromagnetic superconductor RuSr2GdCu2O8 with ferromagnetic Curie temperature TC = 131 K and superconducting transition temperature Tc = 56 K. The low field (20 G) Superconducting hysteresis loop indicates a narrow Meissner state region within average lower critical field Bc1(T)= Bc1(0)[1 - (T/T0)2], with average Bave c1 (0) = 12 G and T0 = 30 K. Full Meissner shielding signal in very low applied field indicates an ab-plane Bab c1(0) ~ 4 G with an estimated anisotropic parameter g ~ 7 for this layered system. The…
We examine energy relaxation of non-equilibrium quasiparticles in different vortex configurations in ``dirty'' $s$-wave superconductors. The heat flow from the electronic subsystem to phonons in a mesoscopic superconducting disk with a radius of the order of several coherence lengths is calculated both in the Meissner and giant vortex states using the Usadel approach. The recombination process is shown to be strongly affected by interplay of the subgap states, located in the vortex core and in the region at the sample edge where the spectral gap $E_{\rm g}$ is reduced by the Meissner currents.…
p-wave superconductors allow for topological defects known as skyrmions, in addition to the usual vortices that are possible in both s-wave and p-wave materials. In strongly type-II superconductors in a magnetic field, a skyrmion flux lattice yields a lower free energy than the Abrikosov flux lattice of vortices, and should thus be realized in p-wave superconductors. We analytically calculate the energy per skyrmion, which agrees very well with numerical results. From this, we obtain the magnetic induction B as a function of the external magnetic field H, and the elastic constants of the skyrm…
In superconducting ferromagnets the equilibrium domain structure is absent in the Meissner state, but appears in the spontaneous vortex phase (the mixed state in zero external magnetic field), though with a period, which can essentially exceed that in normal ferromagnets. Metastable domain walls are possible even in the Meissner state. The domain walls create magnetostatic fields near the sample surface, which can be used for experimental detection of domain walls.
A theoretical description of the influence of surface irregularities (such as wedge-like cracks) on the Bean-Livingston energy barrier is presented. A careful quantitative estimate of the field of the first vortex entry H* into a homogeneous bulk superconductor with crack-like defects is obtained. This estimate is based on an exact analytical solution for the Meissner current distribution in a bulk superconductor with a single two-dimensional infinitely thin crack. This thin crack is characterized by the lowest H* value, and, thus, appears to be the best gate for vortex entry.
We theoretically predict that the critical current $I_c$ and magnetization $M$ of hybrid superconductor/normal-metal (SN) strip may have nonmonotonous dependence on perpendicular magnetic field - so called peak effect. In contrast to familiar peak effect, which is connected either with vortex entry to the superconductor or with peculiarities of vortex pinning, the found phenomenon exists at low fields, in the vortex-free (Meissner) phase. We argue that the effect appears at specific parameters of studied hybrid structure when its in-plane current-supervelocity relation has two maxima. We expec…
We report observation of the nonlinear Meissner effect (NLME) in Nb films by measuring the resonance frequency of a planar superconducting cavity as a function of the magnitude and the orientation of a parallel magnetic field. Use of low power rf probing in films thinner than the London penetration depth, significantly increases the field for the vortex penetration onset and enables NLME detection under true equilibrium conditions. The data agree very well with calculations based on the Usadel equations. We propose to use NLME angular spectroscopy to probe unconventional pairing symmetries in …
The superconducting transition region in a Nb disk showing the paramagnetic Meissner effect (PME) has been investigated in detail. From the field-cooled magnetization behavior, two well-defined temperatures can be associated with the appearance of the PME: T_1 (< T_c) indicates the characteristic temperature where the paramagnetic moment first appears and a lower temperature T_p (< T_1) defines the temperature where the positive moment no longer increases. During the subsequent warming, the paramagnetic moment begins to decrease at T_p and then vanishes at T_1 with the magnitude of the magneti…
We report on the observation of bulk superconductivity from dc magnetization measurements in a cylindrical single crystal of CuxBi2Se3. The magnitude of the magnetization in the Meissner state is very small and the magnetic-field dependence of the magnetization just above the lower critical field Hc1 is very different from those of usual type-II superconductors. We studied the character of the vortex state theoretically in a spin-triplet pairing superconductor and compared it with the experimental results. The results showed that, the superconductivity observed in CuxBi2Se3 is consistent with …
The stress caused by vortices in tetragonal superconductors contributes to the intervortex interaction which depends on vortex orientation within the crystal, on elastic moduli, and is attractive within certain angular regions even in fields along the $c$ crystal axis. For sufficiently strong stress dependence of the critical temperature, this contribution may result in distortions of the hexagonal vortex lattice for $\bm H||\bm c$. In small fields it leads to formation of a square vortex lattice with a fixed $H$ independent spacing. This should be seen in the magnetization $M(H)$ as a discont…
We study the penetration field $H_{\rm P}$ for vortex nanocrystals nucleated in micron-sized samples with edges aligned along the nodal and anti-nodal directions of the d-wave superconducting parameter of Bi$_2$Sr$_2$CaCu$_2$O$_{8 - δ}$. Here we present evidence that the $H_{\rm P}$ for nanocrystals nucleated in samples with edges parallel to the nodal direction is larger than for the antinodal case, $\sim 72$\,\% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce a…
We present small-angle neutron scattering (SANS) and Bitter decoration studies of the superconducting vortices in Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$}. A highly disordered vortex configuration is observed at all measured fields, and is attributed to strong pinning. This conclusion is supported by the absence of a Meissner rim in decoration images obtained close to the sample edge. The field dependence of the magnitude of the SANS scattering vector indicates vortex lattice domains of (distorted) hexagonal symmetry, consistent with the decoration images which show primarily 6-fold coordinated v…
We report muon spin rotation measurements on a single crystal of the marginal type-II superconductor V. The measured internal magnetic field distributions are modeled assuming solutions of the Ginzburg-Landau (GL) equations for an ideal vortex lattice obtained using (i) an iterative procedure developed by E.H. Brandt, Phys. Rev. Lett. 78, 2208 (1997) and (ii) a variational method. Both models yield qualitatively similar results. The magnetic penetration depth (lambda) and the GL coherence length (xi) determined from the data analysis exhibit strong field dependences, which are attributed to ch…
The interrelation between heterogeneity and flux pinning is studied in Ba(Fe1-xCox)2As2 single crystals with widely varying Co-content x. Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x = 0:075 and x = 0:1 reveal highly disordered vortex structures. The width of the Meissner belt observed at the edges of the crystals, and above the surface steps formed by cleaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex intera…
In superconductors, the search for special vortex states such as giant vortices focuses on laterally confined or nanopatterned thin superconducting films, disks, rings, or polygons. We examine the possibility to realize giant vortex states and states with non-uniform vorticity on a superconducting spherical nanoshell, due to the interplay of the topology and the applied magnetic field. We derive the phase diagram and identify where, as a function of the applied magnetic field, the shell thickness and the shell radius, these different vortex phases occur. Moreover, the curved geometry allows th…
The conditions for the entry of vortices into type-II superconductors being in the Meissner and/or mixed state, are studied by both numerical and analytical solution of the Ginzburg-Landau equations. A modulation instability of the superconducting order parameter is shown to occur when the kinematic momentum (or supervelocity) of the condensate reaches a threshold value at the superconductor edge or surface. Due to this instability, vortices start to nucleate at the edge or surface and then penetrate deeper into the sample. It is found that the presence of surface defects causes a noticeable d…
The transverse Meissner effect (TME) in the highly layered superconductor $Bi_2Sr_2CaCu_2O_{8+y}$ with columnar defects is investigated by transport measurements. We present detailed evidence for the persistence of the Bose glass phase for $H_{\perp}<H_{\perp c}$ : (i) the variable-range vortex hopping process for low currents crosses over to the half-loops regime for high currents; (ii) in both regimes near $H_{\perp c}$ the energy barriers vanish linearly with $H_{\perp}$ ; (iii) the transition temperature is governed by $T_{BG}(H_{\parallel},0) -T_{BG}(H_{\parallel},H_{\perp}) \sim |H_{\per…
Starting from the London - type model we study the domain structures in ferromagnetic superconductors taking account of the nucleation of vortices and antivortices coupled to the magnetic texture. We predict that the coupling between domains and vortices results in the formation of two energetically favorable domain configurations: (i) a Meissner - type vortex free configuration with strong domain shrinking and (ii) a more rare domain configuration with a dense vortex -- antivortex lattice. The switching between these configurations is shown to result in the first order phase transitions which…
We present the phase diagram for the current states of superconducting films, based on the experimental investigation of the resistive transition induced by transport current. We found that a rather narrow film never enters the vortex state, but experiences direct transition from the purely superconducting state to the resistive state with phase-slip centers as soon as the current exceeds the Ginzburg-Landau critical current Ic. The Meissner current state of the films of intermediate width transforms at I > 0.8Ic to the vortex resistive state which exists within the current interval 0.8Ic < I …
Local magnetization hysteresis is among the most extensively studied features of high-temperature superconductors (HTSC). The usual source of hysteresis in superconductors is bulk vortex pinning due to material defects. Two additional known mechanisms of magnetic irreversibility are the Bean-Livingston surface barrier and the geometrical barrier (GB). GB arises due to a competition between the line energy of a vortex penetrating into the sample and the Lorentz force of Meissner currents which focuses vortices in the sample's center to form a dome-shaped vortex distribution. This work demonstra…
The discrete vortex lattices in a ferromagnet/superconductor bilayer are studied when the ferromagnet has periodic stripe domains with an out-of-plane magnetization. The vortices are assumed to be situated periodically on chains in the stripe domains. Only up to two vortex chains per domain configurations are considered. When the domain period is fixed, the threshold magnetization is calculated at which the transition from the Meissner state to the mixed state occurs. When the domain period is not fixed, the equilibrium domain size and vortex positions are calculated, depending on the domain's…
A scanning SQUID microscope was used to image vortex trapping as a function of the magnetic induction during cooling in thin-film YBCO strips for strip widths W from 2 to 50 um. We found that vortices were excluded from the strips when the induction Ba was below a critical induction Bc. We present a simple model for the vortex exclusion process which takes into account the vortex - antivortex pair production energy as well as the vortex Meissner and self-energies. This model predicts that the real density n of trapped vortices is given by n=(Ba-BK)/Phi0 with BK = 1.65Phi0/W^2 and Phi0 = h/2e t…
A vortex-glass transition due to point disorder in layered superconductors is studied for the case with an applied field {\it parallel} to the layers. Our calculation of tilt responses indicates that, irrespective of the magnitude of the field, the resulting glass phase, Josephson-vortex-glass (JG), should have a transverse Meissner effect, as in a planar splayed glass phase, only for a tilt perpendicular to the layers. Further, focusing on the high field (and/or high anisotropy) region $B \sqrtΓ > ϕ_0/d^2$, where $Γ$ is the mass anisotropy in the Lawrence-Doniach model, the JG transition line…
Recently it was discovered that the non-uniform Meissner current flowing around the pinning sites in the type-II superconductor induces the unconventional vortex-antivortex pairs with the non-quantized magnetic flux [J.-Y. Ge, et al., Nat. Commun. 6, 6573 (2015)]. Here we provide the theory of this phenomenon showing that the vortex-like structures originate from the perturbation of the current streamlines by the non-superconducting defect, which results in the generation of the localized magnetic field. The position and the shape of such vortex dipoles are shown to be very sensitive to the de…
Anisotropic second-harmonic generation in superconducting nanostructures
Institute of Spintronics and Quantum Information, Faculty of Physics and Astronomy, Adam Mickiewicz University（亚当·密茨凯维奇大学物理与天文学学院自旋电子学与量子信息研究所）；Laboratory of Advanced Quantum Materials and Devices, Institute for Quantum Studies, Chapman University（查普曼大学量子研究学院先进量子材料与器件实验室）
Conventional superconductors are strong diamagnets that through the Meissner effect expel magnetic fields. It would therefore be surprising if a superconducting ground state would support spontaneous magnetics fields. Such time-reversal symmetry broken states have been proposed for the high-temperature superconductors, but their identification remains experimentally controversial. Here we show a route to a low-temperature superconducting state with broken time-reversal symmetry that may accommodate currently conflicting experiments. This state is characterised by an unusual vortex pattern in t…
We describe an experimental protocol to characterize magnetic field dependent microwave losses in superconducting niobium microstrip resonators. Our approach provides a unified view that covers two well-known magnetic field dependent loss mechanisms: quasiparticle generation and vortex motion. We find that quasiparticle generation is the dominant loss mechanism for parallel magnetic fields. For perpendicular fields, the dominant loss mechanism is vortex motion or switches from quasiparticle generation to vortex motion, depending on cooling procedures. In particular, we introduce a plot of the …
We present detailed experimental and theoretical investigations of hotspots produced by trapped vortex bundles and their effect on the radio-frequency (rf) surface resistance $R_s$ of superconductors at low temperatures. Our measurements of $R_s$ combined with the temperature mapping and laser scanning of a 2.36 mm thick Nb plate incorporated into a 3.3 GHz Nb resonator cavity cooled by the superfluid He at 2 K, revealed spatial scales and temperature distributions of hotspots and showed that they can be moved or split by thermal gradients produced by the scanning laser beam. These results, al…
Superconductivity occurs in systems that have a lot of negative charge: the highly negatively charged $(CuO2)^{--}$ planes in the cuprates, negatively charged $(FeAs)^-$ planes in the iron arsenides, and negatively charged $B^-$ planes in magnesium diboride. And, in the nearly filled (with negative electrons) bands of almost all superconductors, as evidenced by their positive Hall coefficient in the normal state. According to the theory of hole superconductivity, metals become superconducting because they are driven to expel negative charge (electrons) from their interior. This is why NIS tunn…
We investigate vortex structure and melting transition in very anisotropic layered superconductors at fields tilted with respect to the c-axis. We show that even a small in-plane field does not homogeneously tilt the vortex lattice but, instead, penetrates inside the superconductor in the form of Josephson vortices (JVs) similar to the Meissner state. At high c-axis magnetic field the phase field of the JV is built up from the phase perturbations created by displacements of pancake vortices. The crossing-lattices ground state leads to linear dependencies of the melting field and melting temper…
We report on the a.c. magnetic response of superconducting UPt_3 in a d.c. magnetic field. At low fields (H < H^*), the in-phase susceptibility shows a sharp drop at $T_c$ followed by a gradual decrease with decreasing temperature, while the out-of-phase component shows a large peak at T_c followed by an unusual broad peak. As the B-C phase line is crossed (H>H^*), however, both the in-phase and out-of-phase susceptibilities resemble the zero-field Meissner curves. We interpret these results in terms of a vortex pinning force which, while comparatively small in the A/B-phases, becomes large en…
