Unconventional Meissner screening induced by chiral molecules in a conventional superconductor
Hen Alpern, Morten Amundsen, Roman Hartmann, Nir Sukenik, Alfredo Spuri, Shira Yochelis, Thomas Prokscha, Vitaly Gutkin, Yonathan Anahory, Elke Scheer, Jacob Li… 展开作者
Hen Alpern, Morten Amundsen, Roman Hartmann, Nir Sukenik, Alfredo Spuri, Shira Yochelis, Thomas Prokscha, Vitaly Gutkin, Yonathan Anahory, Elke Scheer, Jacob Linder, Zaher Salman, Oded Millo, Yossi Paltiel, Angelo Di Bernardo
The coupling of a superconductor to a different material often results in a system with unconventional superconducting properties. A conventional superconductor is a perfect diamagnet expelling magnetic fields out of its volume, a phenomenon known as Meissner effect. Here, we show that the simple adsorption of a monolayer of chiral molecules, which are non-magnetic in solution, onto the surface of a conventional superconductor can markedly change its diamagnetic Meissner response. By measuring the internal magnetic field profile in superconducting Nb thin films under an applied transverse field by low-energy muon spin rotation spectroscopy, we demonstrate that the local field profile inside Nb is considerably modified upon molecular adsorption in a way that also depends on the applied field direction. The modification is not limited to the chiral molecules/Nb interface, but it is long ranged and occurs over a length scale comparable to the superconducting coherence length. Zero-field muon spin spectroscopy measurements in combination with our theoretical analysis show that odd-frequency spin-triplet states induced by the chiral molecules are responsible for the modification of the Meissner response observed inside Nb. These results indicate that a chiral molecules/superconductor system supports odd-frequency spin-triplet pairs due to the molecules acting as a spin active layer and therefore they imply that such system can be used as a simpler alternative to superconductor/ferromagnet or superconductor/topological insulator hybrids for the generation and manipulation of unconventional spin-triplet superconducting states.
It is generally believed that superconducting materials are divided into two classes: `conventional' and `unconventional'. Conventional superconductors (the elements and thousands of compounds including $MgB_2$) are described by conventional London-BCS-Eliashberg electron-phonon theory. There is no general agreement as to what mechanism or mechanisms describe `unconventional' superconductors such as the heavy fermions, organics, cuprate and pnictide families. However all superconductors, whether `conventional' or `unconventional', exhibit the Meissner effect. I argue that there is a single mec…
We investigate the Meissner effect in a parametrically driven superconductor using a semiclassical $U(1)$ lattice gauge theory. Specifically, we periodically drive the $z$-axis tunneling, which leads to an enhancement of the imaginary part of the $z$-axis conductivity at low frequencies if the driving frequency is blue-detuned from the plasma frequency. This has been proposed as a possible mechanism for light-enhanced interlayer transport in YBa$_2$C$_3$O$_{7-δ}$ (YBCO). In contrast to this enhancement of the conductivity, we find that the screening of magnetic fields is less effective than in…
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 proximity coupled topological insulator / superconductor (TI/SC) bilayer system is a representative system to realize topological superconductivity. In order to better understand this unique state and design devices from the TI/SC bilayer, a comprehensive understanding of the microscopic properties of the bilayer is required. In this work, a microwave Meissner screening study, which exploits a high-precision microwave resonator technique, is conducted on the SmB6/YB6 thin film bilayers as an example TI/SC system. The study reveals spatially dependent electrodynamic screening response of th…
Microscopic studies on thin film superconductors play an important role for probing non-equilibrium phase transitions and revealing dynamics at the nanoscale. However, magnetic sensors with nanometer scale spatial and picosecond temporal resolution are essential for exploring these. Here, we present an all-optical, microwave-free method, that utilizes the negatively charged nitrogen-vacancy (NV) center in diamond as a non-invasive quantum sensor and enables the spatial detection of the Meissner state in a superconducting thin film. We place an NV implanted diamond membrane on a superconducting…
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…
A numerical analysis for the Meissner mass in the simplest gluonic phase (the minimal cylindrical gluonic phase II) is performed in the framework of the gauged Nambu-Jona-Lasinio model with cold two-flavor quark matter. We derive Meissner mass formulae without using the numerical second derivative. It is revealed that the gapless mode yields a characterized contribution to the Meissner mass. We also find that there are large and positive contributions from the tree gluon potential term to the transverse modes of gluons. It is shown that the simplest gluonic phase resolves the chromomagnetic in…
Adding Au nanoparticles to YBa$_2$Cu$_3$O$_{7-δ}$ thin films leads to an increase of the superconducting transition temperature $T_{\rm c}$ and the critical current density $j_{\rm c}$. While the higher $j_{\rm c}$ can be understood in terms of a stronger pinning of the flux vortices at the Au nanoparticles, the enhanced $T_{\rm c}$ is still puzzling. In the present study, we determined the microscopic magnetic penetration profiles and the corresponding London penetration depths $λ_{\rm L}$ in the Meissner state of optimally doped YBa$_2$Cu$_3$O$_{7-δ}$ thin films with and without Au nanoparti…
I compute the gluon self-energy in a color superconductor with three flavors of massless quarks, where condensation of Cooper pairs breaks the color and flavor SU(3)_c x U(3)_V x U(3)_A symmetry of QCD to the diagonal subgroup SU(3)_{c+V}. At zero temperature, all eight electric gluons obtain a Debye screening mass, and all eight magnetic gluons a Meissner mass. The Debye as well as the Meissner masses are found to be equal for the different gluon colors. These masses determine the coefficients of the kinetic terms in the effective theory for the low-energy degrees of freedom. Their values agr…
Conventional superconductors respond to external magnetic fields by generating diamagnetic screening currents. However, theoretical work has shown that one can engineer systems where the screening current is paramagnetic, causing them to attract magnetic flux -- a prediction that has recently been experimentally verified. In contrast to previous studies, we show that this effect can be realized in simple superconductor/normal-metal structures with no special properties, using only a simple voltage bias to drive the system out of equilibrium. This is of fundamental interest, since it opens up a…
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.
I compute the gluon self-energy in a color superconductor with two flavors of massless quarks, where condensation of Cooper pairs breaks SU(3)_c to SU(2)_c. At zero temperature, there is neither Debye screening nor a Meissner effect for the three gluons of the unbroken SU(2)_c subgroup. The remaining five gluons attain an electric as well as a magnetic mass. For temperatures approaching the critical temperature for the onset of color superconductivity, or for gluon momenta much larger than the color-superconducting gap, the self-energy assumes the form given by the standard hard-dense loop app…
Empirical surface treatments, such as low-temperature baking (LTB) in a gaseous atmosphere or in vacuum, are important for the surface preparation of Nb superconducting radio frequency (SRF) cavities. These treatments inhomogeneously dope the first $\sim$50 nm of Nb's subsurface and are expected to impart depth-dependent characteristics to its Meissner response; however, direct evidence supporting this remains elusive, suggesting the effect is subtle. In this work, we revisit the Meissner profile data for several LTB treatments obtained from low-energy muon spin rotation (LE-$μ$SR) experiments…
We investigate "hot" regions with anomalous high field dissipation in bulk niobium superconducting radio frequency cavities for particle accelerators by using low energy muon spin rotation (LE-$μ$SR) on corresponding cavity cutouts. We demonstrate that superconducting properties at the hot region are well described by the non-local Pippard/BCS model for niobium in the clean limit with a London penetration depth $λ_\mathrm{L} = 23 \pm 2$ nm. In contrast, a cutout sample from the 120$^\circ$C baked cavity shows a much larger $λ> 100$ nm and a depth dependent mean free path, likely due to gradien…
The odd-frequency superconducting state appears generally in hybrid structures consisting of conventional superconductors and other materials, and features electrons that form temporally non-local Cooper pairs. The quasiclassical theory of superconductivity has been extensively used to model such systems, finding in many cases excellent agreement with experimental measurements. Therefore, it is of interest to study effective models of odd-frequency superconductivity to predict new phenomena associated with this form of unconventional pairing. We establish necessary criteria that the quasiclass…
It was recently pointed out that so-called "superhydrides", hydrogen-rich materials that appear to become superconducting at high temperatures and pressures, exhibit physical properties that are different from both conventional and unconventional standard type I and type II superconductors [1,2]. Here we consider magnetic field expulsion in the first material in this class discovered in 2015, sulfur hydride [3]. A nuclear resonant scattering experiment has been interpreted as demonstration that the Meissner effect takes place in this material [4,5]. Here we point out that the observed effect, …
We examine the effects of nonlocal electrodynamics for a d-wave superconductor on the field dependence of the magnetic penetration depth. The linear field dependence predicted in the local limit, commonly known as the nonlinear Meissner effect, is instead found to be quadratic, $δλ\sim H^2$ for fields below a crossover scale $H^*$. This crossover is shown to be geometry dependent and for most orientations of the screening currents is of the same order as or greater than $H_{c1}$, implying that the nonlinear Meissner effect can not be observed. For special orientations where the current flows a…
The parity breaking gives rise to rich superconducting properties through the admixture of even and odd-parity Cooper pairs. A new light has been shed on parity-breaking superconductors by recent observations of nonreciprocal responses such as the nonlinear optical responses and the superconducting diode effect. In this Letter, we demonstrate that the nonreciprocal responses are characterized by a unidirectional correction to the superfluid density, which we call nonreciprocal superfluid density. This correction leads to the nonreciprocal Meissner effect, namely, asymmetric screening of magnet…
We have measured a paramagnetic Meissner effect in Nb-Al2O3-Nb Josephson junction arrays using a scanning SQUID microscope. The arrays exhibit diamagnetism for some cooling fields and paramagnetism for other cooling fields. The measured mean magnetization is always less than 0.3 flux quantum (in terms of flux per unit cell of the array) for the range of cooling fields investigated. We demonstrate that a new model of magnetic screening, valid for multiply-connected superconductors, reproduces all of the essential features of paramagnetism that we observe and that no exotic mechanism, such as d-…
Recent results about the coexistence of ferromagnetism and unconventional superconductivity with spin-triplet Cooper pairing are reviewed on the basis of the quasi-phenomenological Ginzburg-Landau theory. The superconductivity in the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the spontaneous magnetization. The mixed phase is stable whereas the other superconducting phases that usually exist in unconventional superconductors are either unstable or metastable at relatively low temperatures in a quite narrow domain of the phase diagram and th…
We have directly imaged the anisotropic nonlinear Meissner effect in an unconventional superconductor through the nonlinear electrodynamic response of both (bulk) gap nodes and (surface) Andreev bound states. A superconducting thin film is patterned into a compact self-resonant spiral structure, excited near resonance in the radio-frequency range, and scanned with a focused laser beam perturbation. At low temperatures, direction-dependent nonlinearities in the reactive and resistive properties of the resonator create photoresponse that maps out the directions of nodes, or of bound states assoc…
We simulate two-dimensional Josephson junction arrays, including full mutual- inductance effects, as they are cooled below the transition temperature in a magnetic field. We show numerical simulations of the array magnetization as a function of position, as detected by a scanning SQUID which is placed at a fixed height above the array. The calculated magnetization images show striking agreement with the experimental images obtained by A. Nielsen et al. The average array magnetization is found to be paramagnetic for many values of the applied field, confirming that paramagnetism can arise from …
Non-invasive magnetic field sensing using optically - detected magnetic resonance of nitrogen-vacancy (NV) centers in diamond was used to study spatial distribution of the magnetic induction upon penetration and expulsion of weak magnetic fields in several representative superconductors. Vector magnetic fields were measured on the surface of conventional, Pb and Nb, and unconventional, LuNi$_2$B$_2$C, Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$, and CaKFe$_4$As$_4$, superconductors, with diffraction - limited spatial resolution using variable - temperature confocal sy…
As shown in former papers, the nonadiabatic Heisenberg model presents a novel mechanism of Cooper pair formation which is not the result of an attractive electron-electron interaction but can be described in terms of quantum mechanical constraining forces. This mechanism operates in narrow, roughly half-filled superconducting bands of special symmetry and is evidently responsible for the formation of Cooper pairs in all superconductors. Here we consider this new mechanism within an outer magnetic field. We show that in the magnetic field the constraining forces produce Cooper pairs of non-vani…
The paramagnetic Meissner effect (PME) measured in high $T_{C}$ granular superconductors has been attributed to the presence of $π$-junctions between the grains. Here we present measurements of complex AC magnetic susceptibility from two-dimensional arrays of conventional (non $π$) Nb/Al/AlOx/Nb Josephson junctions. We measured the susceptibility as a function of the temperature $T$, the AC amplitude of the excitation field, $h_{AC}$, and the external magnetic field, $H_{DC}$. The experiments show a strong paramagnetic contribution from the multi-junction loops, which manifests itself as a ree…
This topical review summarizes various features of magnetic penetration depth in unconventional superconductors. Precise measurements of the penetration depth as a function of temperature, magnetic field and crystal orientation can provide detailed information about the pairing state. Examples are given of unconventional pairing in hole- and electron-doped cuprates, organic and heavy fermion superconductors. The ability to apply an external magnetic field adds a new dimension to penetration depth measurements. We discuss how field dependent measurements can be used to study surface Andreev bou…
We consider ferromagnetic particles embedded into a superconductor and study the screening of their magnetic moments by the spins of the Cooper pairs in the superconductor. It is shown that a magnetic moment opposite to the one of the ferromagnetic particle is induced in the superconductor. In the case of a small itinerant ferromagnet grain and low temperatures the full screening of the magnetic moment takes place, \textit{% i.e} the absolute value of the total magnetic moment induced in the superconductor is equal to the one of the ferromagnetic particle. In type II superconductors the propos…
We explore the role of charge reservoir layers (CRLs) on the superconducting transition temperature of cuprate superconductors. Specifically, we study the effect of CRLs with efficient short distance dielectric screening coupled capacitively to copper oxide metallic layers. We argue that dielectric screening at short distances and at frequencies of the order of the superconducting gap, but small compared to the Fermi energy can significantly enhance T$_c$, the transition temperature of an unconventional superconductor. We discuss the relevance of our qualitative arguments to a broader class of…
We present new results for the properties of phases and phase transitions in spin-triplet ferromagnetic superconductors. The superconductivity of the mixed phase of coexistence of ferromagnetism and unconventional superconductivity is triggered by the presence of spontaneous magnetization. The mixed phase is stable but the other superconducting phases that usually exist in unconventional superconductors are either unstable or for particular values of the parameters of the theory some of them are metastable at relatively low temperatures in a quite narrow domain of the phase diagram. Phase tran…
The analisis of Pippard \cite{pip} for the growth of the normal phase into the superconducting phase in the presence of a magnetic field $H>H_c$ is applied in reverse to the case $H<H_c$ ($H_c=$critical magnetic field). We carry out the analysis both for a planar and a cylindrical geometry. As the superconducting phase grows into the normal phase, a supercurrent is generated at the superconductor-normal phase boundary that flows in direction opposite to the Faraday electric field resulting from the moving phase boundary. This supercurrent motion is in direction opposite to what is dictated by …
We derive a general microscopic expression for the non-linear diamagnetic current in a clean superconductor-insulator-normal metal structure with an arbitrary interface transmission. In the absence of electron-electron interactions in the normal metal the diamagnetic response increases monotonously with decreasing temperature showing no sign of paramagnetic reentrance down to T=0. We also analyze the magnetic breakdown of proximity induced Meissner screening. We demonstrate that the magnetic breakdown field should be strongly suppressed in the limit of small interface transmissions while the l…
We present the theoretical approach to study the unconventional Josephson junction (uJJ) made by putting the non-superconduncting strip on the top of superconducting strip. We work in the framework of the Ginzburg-Landau, Bogoliubov de Gennes and Usadel formalisms. We solve the non-linear parital differential equations numerically for few simple cases. We obtaine the eigenenergies of the uJJ by means of combined GL and BdGe method. cases. We predict the occurence of the physical effect, which we call the topological Meissner effect. Basing on the obtained results and current knowledge on Josep…
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…
We calculate the Debye and Meissner masses of photons and gluons for a spin-one color superconductor in the polar and color-spin-locked phases as well as for a spin-zero color superconductor in the 2SC and color-flavor-locked phases. A general derivation for photon-gluon mixing is provided in terms of the QCD partition function. We show qualitatively and quantitatively which of the gauge bosons attain a mass via the Anderson-Higgs mechanism. Contrary to the spin-zero phases, both spin-one phases exhibit an electromagnetic Meissner effect.
The results for the Debye and Meissner screening masses of the gluons and the photon in the case of neutral and beta-equilibrated dense two-flavor quark matter are presented. In the limits of the normal phase and the ideal two-flavor color superconducting phase, the screening masses coincide with the known results. Most interestingly, we find that the Meissner screening masses squared can be negative, indicating a plasma-type chromomagnetic instability in dense quark matter.
Although the diamagnetism is of the essence in the superconducting phenomena, small enough topological (or unconventional) superconductors are paramagnetic. By solving the quasiclassical Eilenberger equation and the Maxwell equation self-consistently in two-dimensional disks, we theoretically study the response of small superconductors to weak magnetic field. Because of the topologically nontrivial character of the wave function, the unconventional superconductor hosts the zero-energy surface Andreev bound states which always accompany the odd-frequency Cooper pairs. The paramagnetic property …
Is superconductivity associated with a lowering or an increase of the kinetic energy of the charge carriers? Conventional BCS theory predicts that the kinetic energy of carriers increases in the transition from the normal to the superconducting state. However, substantial experimental evidence obtained in recent years indicates that in at least some superconductors the opposite occurs. Motivated in part by these experiments many novel mechanisms of superconductivity have recently been proposed where the transition to superconductivity is associated with a lowering of the kinetic energy of the …
We have studied the evolution of the Zero-Bias Conductance Peak (ZBCP) splitting under applied magnetic fields in tunneling experiments on Y1Ba2Cu3O7-x(YBCO), and particular its hysteresis. We have been able to distinguish between two possible contributions to the splitting. One of them is connected to Meissner screening currents whose variation in increasing fields is governed by the Bean-Livingston barrier that delays flux entry well above the lower thermodynamical critical field Hc1, up to the fields of the order of the thermodynamical critical field Hc. The other contribution, dominant in …
We consider an unconventional superconductor in a low frequency harmonic magnetic field. In the Meissner regime at low T a nonlinear magnetic response arises from quasiparticle excitations near minima in the energy gap. Various physical quantities then acquire higher harmonics of the frequency of the applied field. We discuss how examination of the field and angular dependence of these harmonics allows determination of the structure of the energy gap. We show how to distinguish nodes from small finite minima ("quasinodes"). Gaps with nodal lines give rise to universal power law field dependenc…
The Kondo effect in superconductors is frequently investigated using the local quasiparticle density of states as sole bath characteristics, i.e., the presence of anomalous propagators is ignored. Here we point out that this treatment is exact for a number of situations, including point-like impurities in d-wave superconductors. We comment on recent investigations [M. Matsumoto and M. Koga, J. Phys. Soc. Jpn. 70, 2860 (2001), cond-mat/0011256, and Phys. Rev. B 65, 024508 (2002), cond-mat/0103053] which reached different conclusions: while their numerical results are likely correct, their inter…
We point out that the Meissner effect, the process by which a superconductor expels magnetic field from its interior, represents an unsolved puzzle within the London-BCS theoretical framework used to describe the physics of conventional superconductors, because it appears to give rise to non-conservation of angular momentum. Possible ways to avoid this inconsistency within the conventional theory of superconductivity are argued to be farfetched. Consequently, we argue that unless/until a consistent explanation is put forth, the existence of the Meissner effect represents an anomaly that casts …
We show that the behaviour of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductors near a surface is considerably different from the usual case. The order parameter of the FF state is strongly deformed near the surface, which leads to a number of unusual features in the linear magnetic response, such as "anti-screening" or "over-screening" of the applied field. In a fully isotropic FF case, the Meissner effect is still present, despite the vanishing of the transverse superfluid density in the bulk. We also calculate the surface critical field Hc3, which exhibits a peculiar temperature d…
We examine the Meissner state nonlinear electrodynamic effects on the field and angular dependence of the low temperature penetration depth, $λ$, of superconductors in several kinds of unconventional pairing states, with nodes or deep minima (``quasinodes'') in the energy gap. Our calculations are prompted by the fact that, for typical unconventional superconducting material parameters, the predicted size of these effects for $λ$ exceeds the available experimental precision for this quantity by a much larger factor than for others. We obtain expressions for the nonlinear component of the penet…
We present a new measurement method which can be used to image gap nodal structure of superconductors whose pairing symmetry is under debate. This technique utilizes a high quality factor microwave resonance involving the sample of interest. While supporting a circularly symmetric standing wave current pattern, the sample is perturbed by a scanned laser beam, creating a photoresponse that was previously shown to reveal the superconducting gap anisotropy. Simulation and the measurement of the photoresponse of an unpatterned Nb film show less than 8\% anisotropy, as expected for a superconductor…
We measured the initial M-H curves for a sample of the newly discovered superconductor NdFeAsO0.82Fe0.18, which had a critical temperature, Tc, of 51 K, and was fabricated at the high pressure of 6 GPa. The lower critical field, Hc1, was extracted from the deviation point of the Meissner linearity in the M-H curves, which show linear temperature dependence in the low temperature region down to 5 K. The Hc1(T) indicates no s-wave superconductivity, but rather an unconventional superconductivity with a nodal gap structure. Furthermore, the linearity of Hc1 at low temperature does not hold at hig…
