Diamagnetic Response Research Articles

Role of electronic excitation on the anomalous magnetism of elemental copper

The magnetic susceptibility of elemental copper (Cu) shows an anomalous rise at low temperatures superimposed on the expected atypical diamagnetic response. Such temperature-dependent susceptibility, which is also known as the Curie tail, cannot be explained on the basis of the Larmor diamagnetic and Pauli paramagnetic contributions expected in Cu. Using valence band resonant photoemission spectroscopy results and density functional theory calculations, we show the magnetic anomaly appears due to the presence of holes in the Cu $3d$ band, which originates from a thermally excited electronic configuration. Our study therefore highlights that the Curie tail, which is generally overlooked presuming it is either due to paramagnetic impurities or defects, can in fact be intrinsic to a material, and even simple systems such as elemental Cu are susceptible to electronic excitations giving rise to an anomalous magnetic state.

Spontaneous orbital magnetization of mesoscopic dipole dimers

Ensembles of gold nanoparticles present a magnetic behavior which is at odds with the weakly diamagnetic response of bulk gold. In particular, an unusual ferromagnetic order has been unveiled by several experiments. Here we investigate if the combined effect of orbital magnetism of conduction electrons and interparticle dipolar interaction can lead to magnetic ordering. Using different model systems of interacting mesoscopic magnetic dipoles, together with a microscopic description of the electron dynamics within the nanoparticles, we find that a spontaneous magnetic moment may arise in dimers of metallic nanoparticles when the latter are characterized by a large orbital paramagnetic susceptibility.

Magnetic response of metallic nanoparticles: Geometric and weakly relativistic effects

While the large paramagnetic response measured in certain ensembles of metallic nanoparticles has been assigned to orbital effects of conduction electrons, the spin-orbit coupling has been pointed out as a possible origin of the anomalously large diamagnetic response observed in other cases. Such a relativistic effect, arising from the inhomogeneous electrostatic potential seen by the conduction electrons, might originate from the host ionic lattice, impurities, or the self-consistent confining potential. Here we theoretically investigate the effect of the spin-orbit coupling arising from the confining potential, quantifying its contribution to the zero-field magnetic susceptibility and gauging it against the ones generated by other weakly-relativistic corrections. Two ideal geometries are considered in detail, the sphere and the half-sphere, focusing on the expected increased role of the spin-orbit coupling upon a symmetry reduction, and the application of these results to actual metallic nanoparticles is discussed. The matrix elements of the different weakly-relativistic corrections are obtained and incorporated in a perturbative treatment of the magnetic field, leading to tractable semi-analytical and semiclassical expressions for the case of the sphere, while a numerical treatment becomes necessary for the half-sphere. The correction to the zero-field susceptibility arising from the spin-orbit coupling in a single sphere is quite small, and it is dominated by the weakly-relativistic kinetic energy correction, which in turn remains considerably smaller than the typical values of the nonrelativistic zero-field susceptibility. Moreover, the spin-orbit contribution to the average response for ensembles of nanoparticles with a large size dispersion is shown to vanish. The symmetry reduction in going from the single sphere to the half-sphere does not translate into a significant (...)

Detection of graphene's divergent orbital diamagnetism at the Dirac point.

The electronic properties of graphene have been intensively investigated over the past decade. However, the singular orbital magnetism of undoped graphene, a fundamental signature of the characteristic Berry phase of graphene’s electronic wave functions, has been challenging to measure in a single flake. Using a highly sensitive giant magnetoresistance (GMR) sensor, we have measured the gate voltage–dependent magnetization of a single graphene monolayer encapsulated between boron nitride crystals. The signal exhibits a diamagnetic peak at the Dirac point whose magnetic field and temperature dependences agree with long-standing theoretical predictions. Our measurements offer a means to monitor Berry phase singularities and explore correlated states generated by the combined effects of Coulomb interactions, strain, or moiré potentials.

Development of high-resolution capacitive Faraday magnetometers for sub-Kelvin region.

High-sensitivity capacitive Faraday magnetometers were developed for static DC magnetization measurements in a sub-Kelvin region that can be used with 3He-4He dilution refrigerators (∼50 mK) and 3He refrigerators (∼0.28K). For high-resolution magnetization measurements, the background magnetization of the force-sensing capacitor should be as small as possible, compared with the magnetization value of a measured specimen. In this study, we succeeded in reducing the background of the capacitor in both low- and high-field regions by compensating for the diamagnetic response of a thin quartz plate, making use of Pauli-paramagnetic alloys and Van Vleck paramagnets as a counter magnetization for a diamagnetic signal. Having established an ultra-high-sensitivity capacitor, we achieved a resolution of 10-5 (∼10-5-10-6) emu in the low- (high-) field region below (above) 1T. In particular, the newly developed capacitors with a Van Vleck paramagnet Pr0.1La0.9Be13 and paramagnetic MgAl alloys are demonstrated to be very useful for high-resolution magnetization measurements at milli-Kelvin temperatures in low and high magnetic fields, respectively.

Unconventional Meissner screening induced by chiral molecules in a conventional superconductor

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.

High temperature superconductivity at FeSe/LaFeO3 interface

Enormous enhancement of superconducting pairing temperature (Tg) to 65 K in FeSe/SrTiO3 has made it a spotlight. Despite the effort of interfacial engineering, FeSe interfaced with TiOx remains the unique case in hosting high Tg, hindering a decisive understanding on the general mechanism and ways to further improving Tg. Here we constructed a new high-Tg interface, single-layer FeSe interfaced with FeOx-terminated LaFeO3. Large superconducting gap and diamagnetic response evidence that the superconducting pairing can emerge near 80 K, highest amongst all-known interfacial superconductors. Combining various techniques, we reveal interfacial charge transfer and strong interfacial electron-phonon coupling (EPC) in FeSe/LaFeO3, showing that the cooperative pairing mechanism works beyond FeSe-TiOx. Intriguingly, the stronger interfacial EPC than that in FeSe/SrTiO3 is likely induced by the stronger interfacial bonding in FeSe/LaFeO3, and can explain the higher Tg according to recent theoretical calculations, pointing out a workable route in designing new interfaces to achieve higher Tg.

Ultraweakly and fine-tunable negative permittivity of polyaniline/nickel metacomposites with high-frequency diamagnetic response

Metacomposites have drawn considerable attention due to their enticing prospect in electronic and dielectric devices, which meanwhile unfortunately suffered from extremely high negative permittivity (ε' < 0). Herein, we put forward a strategy to suppress the plasma oscillation in metacomposites and thereby constrain the value of negative permittivity by introducing the semiconductive polyaniline to construct polyaniline/nickel (PANI/Ni) metacomposites. Weakly negative permittivity was obtained at a magnitude of 102 over the whole test frequency band, which is remarkably lower by at least three orders of magnitude than that of most metacomposites. The ε′-negative value was resultantly fine-tuned by about 102 with equally adjusting Ni content. Lorentz and Drude models were employed to describe ε′-negative spectra, indicating that both dielectric resonance and plasma oscillation contributed to negative permittivity. Negative susceptibility of ε′-negative materials was mainly ascribed to high-frequency diamagnetic response within Ni networks. The well-designed PANI/Ni metacomposites with weakly and fine-tuning negative permittivity can greatly benefit the practical applications on electromagnetic (EM) shielding, microwave absorption and novel EM sensors.

Diamagnetic and paramagnetic Meissner effect from odd-frequency pairing in multiorbital superconductors

The Meissner effect is one of the defining properties of superconductivity, with a conventional superconductor completely repelling an external magnetic field. In contrast to this diamagnetic behavior, odd-frequency superconducting pairing has often been seen to produce a paramagnetic Meissner effect, which instead makes the superconductor unstable due to the attraction of magnetic field. In this work we study how both even- and odd-frequency superconducting pairing contributes to the Meissner effect in a generic two-orbital superconductor with a tunable odd-frequency pairing component. By dividing the contributions to the Meissner effect into intra- and inter-band processes, we find that the odd-frequency pairing actually generates both dia- and paramagnetic Meissner responses, determined by the normal-state band structure. More specifically, for materials with two electron-like (hole-like) low-energy bands we find that the odd-frequency inter-band contribution is paramagnetic but nearly canceled by a diamagnetic odd-frequency intra-band contribution. Combined with a diamagnetic even-frequency contribution, such superconductors thus always display a large diamagnetic Meissner response to an external magnetic field, even in the presence of large odd-frequency pairing. For materials with an inverted, or topological, band structure, we find the odd-frequency inter-band contribution to instead be diamagnetic and even the dominating contribution to the Meissner effect in the near-metallic regime. Taken together, our results show that odd-frequency pairing in multi-orbital superconductors does not generate a destabilizing paramagnetic Meissner effect and can even generate a diamagnetic response in topological materials.

Extraordinary kinetic inductance of superconductor/ferromagnet/normal metal thin strip in an Fulde–Ferrell state

We have calculated kinetic inductance L k of a thin superconductor/ferromagnet/normal metal strip in an in-plane Fulde–Ferrell (FF) state. We consider range of parameters when FF state appears at temperature T FF < T c (T c is a transition temperature to superconducting state) when the paramagnetic response of FN layers overcomes the diamagnetic response of S layer. We show that L k diverges at T = T FF which is consequence of the second order phase transition to FF state, similar to divergency of L k at T = T c. Kinetic inductance also diverges at finite magnetic field at T < T FF which is consequence of magnetic field driven second order transition to/from FF state. Due to presence in the FF state finite supervelocity, at low current there are two states (metastable and ground) which have different L k. Metastable state is unstable above some critical current which is much smaller than depairing current, above which the ground state becomes unstable. It results in strong dependence of L k on current not only at large currents (near depairing current) but at low currents too. We argue that found properties could be useful in various applications which exploit temperature, current and magnetic field dependence of the kinetic inductance.

Saddle point anomaly of Landau levels in graphenelike structures

Studying the tight-binding model in an applied rational magnetic field $(H)$ we show that in graphene there are very unusual Landau levels situated in the immediate vicinity of the saddle point ($M$ point) energy ${\ensuremath{\epsilon}}_{M}$. Landau levels around ${\ensuremath{\epsilon}}_{M}$ are broadened into minibands (even in relatively weak magnetic fields, $\ensuremath{\sim}40--53$ T), with the maximal width reaching 0.4--0.5 of the energy separation between two neighboring Landau levels, though at all other energies the width of Landau levels is practically zero. In terms of the semiclassical approach a broad Landau level or magnetic miniband at ${\ensuremath{\epsilon}}_{M}$ is a manifestation of the so-called self-intersecting orbit signifying an abrupt transition from the semiclassical trajectories enclosing the $\mathrm{\ensuremath{\Gamma}}$ point to the trajectories enclosing the $K$ point in the momentum space. The structure of broad magnetic minibands is clearly fractal, demonstrating fast oscillations of the band energy $E(k)$ with the magnetic wave vector $k$. Remarkably, the saddle point virtually does not affect the diamagnetic response of graphene, which is caused mostly by electron states in the vicinity of the Fermi energy ${\ensuremath{\epsilon}}_{F}$. Experimentally, the effect of the broadening of Landau levels can possibly be observed in twisted graphene where two saddle point singularities can be brought close to the Fermi energy. At zero temperature the longitudinal conductivity is expected to be proportional to the calculated magnetic density of states at the Fermi level.

NMR determination of Van Hove singularity and Lifshitz transitions in the nodal-line semimetal ZrSiTe

We have applied nuclear magnetic resonance spectroscopy to study the distinctive network of nodal lines in the Dirac semimetal ZrSiTe. The low-$T$ behavior is dominated by a symmetry-protected nodal line, with NMR providing a sensitive probe of the diamagnetic response of the associated carriers. A sharp low-$T$ minimum in NMR shift and $(T_1T)^{-1}$ provides a quantitative measure of the dispersionless, quasi-2D behavior of this nodal line. We also identify a van Hove singularity closely connected to this nodal line, and an associated $T$-induced Lifshitz transition. A disconnect in the NMR shift and line width at this temperature indicates the change in electronic behavior associated with this topological change. These features have an orientation-dependent behavior indicating a field-dependent scaling of the associated band energies.

Magnetism in graphene flakes with edge disorder

The magnetization of graphene flakes as a function of size, shape, boundary type, and edge defects is computed in a tight-binding model. Flakes with Klein boundaries exhibit a smaller orbital magnetization than flakes with other boundaries. The difference in magnitude is significant for flakes with a few hundred to a few thousand atoms. One can tune the magnetization of a zigzag or Klein flake quasicontinuously by adding atoms, one by one, to the edges of the zigzag flake, or removing atoms from a Klein flake. Flakes with an odd number of atoms show a paramagnetic spin response due to particle-hole symmetry. The addition of a next-nearest neighbor term to the Hamiltonian, which breaks particle-hole symmetry, does not destroy this effect as long as there is approximate particle-hole symmetry. Other defects that affect the chemical potential, such as edge atoms with an onsite potential, or doping, can affect the paramagnetism but preserve the difference in magnetization between Klein and non-Klein flakes. These results are consistent with experiments which see paramagnetic response at low magnetic fields crossing over to diamagnetic response at higher fields.

Moiré lattice effects on the orbital magnetic response of twisted bilayer graphene and Condon instability

We analyze the orbital magnetic susceptibility from the band structure of twisted bilayer graphene. Close to charge neutrality, the out-of-plane susceptibility inherits the strong diamagnetic response from graphene. Increasing the doping, a crossover from diamagnetism to paramagnetism is obtained and a logarithmic divergence develops at the van Hove singularity of the Moir\'e lattice in the first band. The enhanced paramagnetism at the van Hove singularity is stronger for relatively large angle but gets suppressed by the flat spectrum towards the vicinity of the first magic angle. A diverging paramagnetic susceptibility indicates an instability towards orbital ferromagnetism with an orbital out-of-plane magnetization and a Landau level structure. The region of instability is however found to be practically very small, parametrically suppressed by the ratio of the electron velocity to the speed of light. We also discuss the in-plane orbital susceptibility at charge neutrality where we find a paramagnetic response and a logarithmic divergence at the magic angle. The paramagnetic response is associated with negative counterflow current in the two layers and does not admit a semiclassical description.

Radio-frequency epsilon-negative property and diamagnetic response of percolative Ag/CCTO metacomposites

Here, Ag/CaCu3Ti4O12 (Ag/CCTO) percolative nanocomposites were fabricated by an in-situ synthesis, where the percolation-related transitions of dielectric and magnetic properties were investigated at radio-frequency band. Epsilon-negative property (ε′ < 0) and diamagnetic responses (μ′ < 1) were observed in percolated composites, owing to low-frequency plasmonic state and induced eddy currents within formed conductive Ag networks respectively. The competitive effect between polarization response of ε′-positive CCTO matrix and collective plasma oscillation of ε′-negative Ag networks leads to the tunable negative permittivity. The clarification of inductive character of ε′-negative materials benefits constructing effective strategy for adjusting negative value of dielectric constant.

Tuning of superdiamagnetism in La2CuO4 by solid-state electrochemical fluorination and defluorination

Here, we demonstrate the electrochemical fluorination of La2CuO4 in an all-solid-state cell. This method of fluorine intercalation is controllable and reproducible, offering a precise adjustment of hole doping and thus tuning of superdiamagnetic (i.e., the perfect diamagnetic behavior of a superconductor) properties. The fluorinated La2CuO4Fx samples showed an increase in Tc and in diamagnetic response with increasing fluorine content with x up to ∼0.2. The fluorination process could also be reversed, as fluorine could be electrochemically deintercalated from La2CuO4Fx under re-formation of the antiferromagnetic insulator La2CuO4, returning the samples to a non-superdiamagnetic state. This method offers a convenient way of studying the detailed effects of hole doping in La2CuO4 and shows that tuning of material properties by electrochemical fluorination can also be extended to the field of superconductors.

Percolated cermets of nickel/yttrium iron garnet for double negative metacomposites

Dielectric and magnetic properties of percolative nickel/yttrium iron garnet cermets were investigated. With nickel content increasing, microstructure evolution of the cermets exhibited characteristics of percolation, resulting in electrical conductivity increased dramatically and conduction mechanism changed from hopping to metallic behaviors. Negative permittivity and negative permeability were observed. Negative permittivity was caused by the plasmonic state of delocalized electrons. Negative permeability was attributed to magnetic resonance of ferromagnetic constituents and diamagnetic responses of eddy currents induced in interconnected nickel networks. The obtained double negative metacomposites enriched metamaterials and have great potential in electromagnetic shielding, cloaking, and attenuation applications.

Transport of charge carriers across the normal-superconducting interfaces in Bi1.65Pb0.35Sr2Ca2Cu3O10+δ nanoceramics

Samples of superconducting Bi1.65Pb0.35Sr2Ca2 Cu3O10+δ (Bi-2223) with nanoparticle-sized grains were produced with the help of high-energy ball milling, followed by pelletizing, and flash sintering. X-ray powder diffraction patterns suggest that the grain size was not altered by flash sintering. The temperature dependence of the magnetic susceptibility χ(T), electrical resistivity ρ(T), and thermoelectric potential S(T), all revealed features consistent with superconductivity below Tc≈ 108 K, though these features were progressively suppressed with the milling time. These data indicate that milling generates nanoparticle-sized grains with the Bi-2223 phase at the core, surrounded by a severely damaged and disordered outer shell. The thickness of this outer layer increases with milling time. The extent of the damage surrounding the grain cores has severe implications on the properties of the pellets. While the onset of superconductivity is clearly marked by the diamagnetic response in χ(T), and a drop in S(T), the signature in ρ(T) is more subtle and difficult to resolve, as it is superimposed on a semiconducting-like resistivity background. Measurements of the real (ε′) and imaginary (ε″) components of the dielectric permittivity in the low frequency limit reveal two distinct interfaces of charge accumulation, one at the margins between the crystalline grain cores and the damaged outer layers, and the second between the grain boundaries. The behaviors of ρ(T) and ε″ near room temperature follow closely an Arrhenius dependence, yielding activation energies in the range between 50 and 200 meV. The transport properties can be explained by a hopping model.

Optimum heat treatment to enhance the weak-link response of Y123 nanowires prepared by Solution Blow Spinning

Although the production of YBa2Cu3O7-δ (Y123) has been extensively reported, there is still a lack of information on the ideal heat treatment to produce this material in the form of one dimension nanostructures. Thus, by means of the Solution Blow Spinning technique, metals embedded in polymer fibers were prepared. These polymer composite fibers were fired and then investigated by thermogravimetric analysis. The maximum sintering temperatures of heat treatment were chosen in the interval 850 °C–925 °C for 1 h under oxygen flux. SEM images allowed us to determine the wire diameter as approximately 350 nm for all samples, as well as to map the evolution of the entangled wire morphology with the sintering temperature. XRD analysis indicated the presence of Y123 and secondary phases in all samples. Ac magnetic susceptibility and dc magnetization measurements demonstrated that the sample sintered at 925 °C/1 h is the one with the highest weak-link critical temperature and the largest diamagnetic response.

Intermediate type-I superconductors in the mesoscopic scale

M. Tinkham and P. G. de Gennes, described in their books the existence of an intermediate type-I superconductor as a consequence of an external surface that affects the well known classification of superconductors into type-I and II. Here we consider the mesoscopic superconductor where the ratio volume to area is small and the effects of the external surface are enhanced. By means of the standard Ginzburg-Landau theory the Tinkham-de Gennes scenario is extended to the mesoscopic type-I superconductor. We find new features of the transition at the passage from the genuine to the intermediate type-I. The latter has two distinct transitions, namely, from a paramagnetic to diamagnetic response in descending field and a quasi type-II behavior as the critical coupling is approached in ascending field. The intermediate type-I phase proposed here, and its corresponding transitions, reflect intrinsic features of the superconductor and not its geometrical properties.