We analyze residual frozen flux in a long narrow superconducting strip cooled through its transition temperature T_c in a small perpendicular magnetic field. This problem is relevant for the issue of trapped magnetic flux in superconducting electronic devices. During cooling, the low-temperature vortex configuration is formed at temperatures very close to T_c, where the flux density is determined by dynamic balance between the thermally-activated exits and entries of vortices over the geometrical energy barrier formed by the interaction with the strip edges and the Meissner screening current. In the field range between the minimum flux-expulsion field and the penetration field, the equilibrium flux density is finite due to thermal activation and rapidly decreases with decreasing temperature. During cooling, however, the escape rate decreases exponentially, and the vortex density falls out of equilibrium at a field-dependent freezing temperature T_fr. We derive and solve the dynamic-balance equation for this process, which yields definite quantitative results for T_fr and the frozen vortex density. The relative freezing temperature 1-T_fr/T_c exceeds the fluctuation width of the transition by a large logarithmic factor, rapidly increases when the magnetic field approaches the minimum flux-expulsion field, and logarithmically increases with decreasing cooling rate. The resulting frozen flux density has a very strong magnetic-field dependence which can be used to define the effective flux-expulsion magnetic field.

The discovery of iron-based superconductors proves that high-temperature superconductivity is not limited to cuprates. Here, we use transport measurements to determine the in-plane anisotropy of the upper critical field (Hc2) in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence of the Hc2 anisotropy for fields along inter-planar and in-plane directions was found to increase in the over-doped regime. For underdoped samples with a spin-nematic phase below the tetragonal-to-orthorhombic structural transition temperature, we find that Hc2 along the a axis is considerably lower than that along the b axis. This Hc2 anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with previous works, we conclude that superconductivity in underdoped iron pnictides is orbital selective—with a dominant contribution from electrons with the dyz orbital character and from being intimately associated with spin excitations.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including BaFe_2As_2, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic (AF) ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned BaFe_2As_2 and compare the results to the non-magnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, BaFe_2As_2 exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in BaFe_2As_2 that is not present in the Ni-doped material.

We report an unusual anisotropic paramagnetic peak effect observed in reversible magnetization of a single crystalline nodal superconductor Rh_17S_15. Both temperature- and field-dependent magnetization measurements reveal a distinct novel vortex state above approximately 1 T. This peak effect is most pronounced when the magnetic field, H, is applied parallel to the [111] direction, whereas it diminishes for H∥[110]. Intriguingly, for H∥[100], instead of a peak, we observe a step-like decrease in M(T), with the step amplitude increasing in larger applied magnetic fields. This behavior is opposite to the expectations of conventional Meissner expulsion. The magnitude of the peak effect, expressed in terms of dimensionless volume susceptibility, is on the order of Δχ=10^-5 (with full diamagnetic screening corresponding to χ=-1). The observed anisotropic paramagnetic vortex response is unusual considering the cubic symmetry of Rh_17S_15. We propose that in this distinct vortex phase, a small but finite attractive interaction between vortices below H_c2 may be responsible for this unusual phenomenon. Furthermore, the vortices seem to prefer aligning along the [111] direction, rotating toward it when the magnetic field is applied in other directions. Our findings add another item to the list of unusual properties of Rh_17S_15 that attracted recent attention as the first unconventional superconductor that has a mineral analog, miassite, found in nature.

Multiple anomalous features in electronic spectra of metals with a kagome lattice structure-van Hove singularities, Dirac points, and flat bands-imply that materials containing this structural motif may lie at a nexus of topological and correlated electron physics. Due to the prospects of such exceptional electronic behavior, the recent discovery of superconductivity coexisting with charge-density wave (CDW) order in the layered kagome metals A V 3 Sb 5 (A A = K,Rb,Cs) has attracted considerable attention. Notably, these archetypal kagome metals express unconventional magnetotransport behavior, including an unexpected linear-in-H H diagonal resistivity at low fields, and an even more peculiar, nonmonotonic sign-changing behavior of the Hall resistivity, which has been speculated to arise from a chiral CDW. We argue here that this unusual magnetotransport derives not from such unconventional phenomena, but rather from the unique fermiology of the A V 3 Sb 5 materials. Specifically, it is caused by a large, concave hexagonal Fermi surface sheet formed in the close proximity to the van Hove singularities, which is backfolded into a small hexagonal sheet and two large triangular sheets in the CDW state. We introduce and analyze a model of the electronic structure of these Fermi surface sheets that allows for a full analytical treatment within Boltzmann kinetic theory and that enables semi-quantitative fits of our transport data. Specifically, we find that the anomalous magnetotransport behavior is caused by the confluence of strong reduction of the Fermi velocity near the van Hove singularities located near the vertices of the hexagonal sheet and sharp corners in Fermi surface generated by the CDW reconstruction. Our analytical approach not only explains the anomalous magnetotransport in the kagome superconductors but also can be extended to a variety of metallic systems hosting singular features in their Fermi surfaces.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge -spin interactions in the system. Although the parent state of iron -based high -temperature superconductors, including BaFe2As2, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in -plane MR effect due to twin domains in the orthorhombic antiferromagnetic ordered state. Here, we study the in -plane magnetoresistance anisotropy in detwinned BaFe2As2 and compare the results to the nonmagnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, BaFe2As2 exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge -spin interaction in BaFe2As2 that is not present in the Ni-doped material.

Van Hove singularities (vHs) located close to the Fermi level in Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have profound influence on their electronic and transport characteristics. Specifically, magneto-transport and susceptibility measurements on CsV3Sb5 reveal an anomalous temperature dependence of the upper critical field H_c2 (T), characterized by a pronounced upward curvature for both in-plane and c-axis magnetic fields, with zero-temperature H_c2 values of 6.0 T and 1.2 T, respectively. Our theoretical analysis, using a newly developed single-band model incorporating vHs and gap anisotropy, suggests that the observed upper critical field behavior is predominantly driven by the anisotropy of the Fermi velocity originating from vHs, instead of multi-band effects or gap anisotropy. Increased electron scattering introduced by proton irradiation defects smears out the vHs, reduces anisotropy, and recovers the conventional H_c2 (T) behavior, corroborating our proposed model.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and compare the results to the nonmagnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ that is not present in the Ni-doped material.

We use transport measurements to determine the in-plane anisotropy of the upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence (x) of the upper critical field anisotropy for fields along the inter-planar (c-axis) and in-plane field directions was found to increase in the overdoped regime. For underdoped samples, which exhibit a spin nematic phase below the tetragonal to orthorhombic structural transition temperature Ts , we find that Hc2 along the a-axis is considerably lower than that along the b-axis. The upper critical field anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with inelastic neutron scattering studies of spin excitations, and angle-resolved photoemission spectroscopy, we conclude that superconductivity in under-doped iron pnictides is orbital selective - with a dominant contribution from electrons with the dyz orbital character and being intimately associated with spin excitations.

Mesa-shaped structures of the high critical temperature (high-T-c) superconductor Bi2Sr2CaCu2O8+delta; contain stacked intrinsic Josephson junctions. As such, they are a promising source of coherent radiation in the "terahertz gap" range, spanning from approximately 0.3 to 2.0 THz. Technological applications of these devices become far more practical if they can be operated at a cryogenic bath temperature of 77 K or higher. Previous works have reported emission from this type of device at high terahertz power levels at lower operating temperatures, 40-60 K, while at T-bath >= 77 K observed power levels have gen-erally been low. Here we report generation of 130 mu W of coherent power at 0.456 THz from a mesa of Bi2Sr2CaCu2O8+delta; doped with 0.16 holes per Cu atom, at a bath temperature of 77.4 K. We find that the device radiates terahertz power when clearly identifiable cavity modes are excited, and that the frequency and bias voltage corresponding to each of these modes is almost independent of temperature. This is con-sistent with these modes having terahertz-frequency electric fields with very little dependence on vertical position within the mesa. We also find that the terahertz power radiated from any given mode decreases monotonically as the mesa temperature is increased. On the other hand, the low-frequency modes become inaccessible at low temperatures due to retrapping of the intrinsic Josephson junctions, and the maximum radiation power for the emitting mode is typically achieved at the temperature at which the retrapping voltage reaches the resonance voltage for this mode.

We have investigated the intermediate layer between the gallium-gadolinium garnet substrate and the yttriumiron garnet epitaxial film. This interlayer is characterized by a large magnetic moment at low temperatures, which leads to an anomalous dependence of the susceptibility and a shift in the magnetic resonance frequency upon cooling. This shift is much larger than might be expected due to magnetization anisotropy, substrate paramagnetic ions, and magnetostriction. We found that this interlayer plays a major role in magnetic relaxation processes in the temperature range below 50 K.

We report the synthesis and characterization of phase pure Ta 3 Sb, a material predicted to be topological with eightfold degenerate fermionic states [Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Sci. Adv. 5, eaav8575 (2019]. We observe superconductivity in Ta 3 Sb with 𝑇 𝑐 ~ 0.67 K in both electrical resistivity ρ( T ) and specific heat C ( T ) measurements. Field dependent measurements yield the superconducting phase diagram with an upper critical field of 𝐻 𝑐2 (0)~ 0.95 T, corresponding to a superconducting coherence length of ξ  18.6 nm. The gap ratio deduced from specific heat anomaly, 2∆ 0 /𝑘 𝐵 𝑇 𝑐 is 3.46, a value close to the Bardeen-Cooper-Schrieffer (BCS) value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of κ  90 is obtained identifying Ta 3 Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta 3 Sb as an appealing platform to further explore exotic quantum states in multifold

We explore a phenomenological phase diagram for the magnetic helical state with 90 degrees turn angle between neighboring spins in the external magnetic field. Such a state is formed by the Eu spin layers in the superconducting iron arsenide RbEuFe4As4. The peculiarity of this spin configuration is that it is not realized in the standard Heisenberg model with bilinear exchange interactions. A minimum model allowing for such a state requires the biquadratic nearest-neighbor interaction term. In addition, in tetragonal materials, the 90 degrees helix state may be stabilized by the in-plane fourfold anisotropy term, which also fixes helix orientation with respect to the crystal lattice. Such a system has a very rich behavior in the external magnetic field. The magnetic field induces the metamagnetic transition to the double-periodic state with the moment angles (alpha, alpha, -alpha, -alpha) with respect to the field for the four subsequent spins. The transition field to this state from the deformed helix is determined by the strength of biquadratic interaction. The transition is second-order for small biquadratic coupling and becomes first-order when this coupling exceeds the critical value. On the other hand, the aligned state at high magnetic field becomes unstable with respect to the formation of an incommensurate fan state, which transforms into the double-periodic state with decreasing magnetic field. The range of this incommensurate state near the saturation field is proportional to square of the biquadratic coupling. In addition, when the magnetic field is applied along one of four the equilibrium moment directions, the deformed helix state experience the first-order rotation transition at the field determined by the fourfold anisotropy.

We evaluate the spin-wave spectrum and dynamic susceptibility in a layered superconductor with helical interlayer magnetic structure. We especially focus on the structure in which the moments rotate 90 ◦ from layer to layer realized in the iron pnictide RbEuFe 4 As 4 . While in nonmagnetic superconductors low-frequency magnetic field decays on the distance of the order of the London penetration depth, spin waves mediate its propagation to much larger distances limited by external dissipation mechanisms. The spin-wave spectrum in superconductors is strongly renormalized due to the long-range electromagnetic interactions between the oscillating magnetic moments. This leads to strong enhancement of the frequency of the mode coupled with uniform field and this enhancement exists only within a narrow range of the c -axis wave vectors of the order of the inverse London penetration depth. The key feature of materials like RbEuFe 4 As 4 is that this uniform mode corresponds to the maximum frequency of the spin-wave spectrum with respect to the c -axis wave vector. As a consequence, the high-frequency surface resistance acquires a very distinct asymmetric feature spreading between the bare and renormalized frequencies. We also consider excitation of spin waves with the Josephson effect in a tunneling contact between helical-magnetic and conventional superconductors and study the interplay between the spin-wave features and geometrical cavity resonances in the current-voltage characteristics.

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe_{4}As_{4} in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. These results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe_{4}As_{4}, with important implications for future investigations of physical phenomena arising from the interplay between them.

We consider a clean layered magnetic superconductor in which a continuous magnetic transition takes place inside superconducting state and the exchange interaction between superconducting and magnetic subsystems is weak so that superconductivity is not destroyed at the magnetic transition. An example of such material is RbEuFe$_{4}$As$_{4}$. We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations in the vicinity of the magnetic transition. The influence of nonuniform exchange field on superconducting parameters is sensitive to the relation between the magnetic correlation length, $\xi_{h}$, and superconducting coherence length $\xi_{s}$ defining the 'scattering' ($\xi_{h} \xi_{s}$) regimes. As a small uniform exchange field does not affect the superconducting gap and superfluid density at zero temperature, smoothening of the spatial variations of the exchange field reduces its effects on these parameters. We develop a quantitative description of this 'scattering-to-smooth' crossover for the case of quasi-two-dimensional magnetic fluctuations. Since the magnetic-scattering probability varies at the energy scale comparable with the gap, the quasiclassical approximation is not applicable in the crossover region and microscopic treatment is required. We find that the corrections to both the gap and superfluid density grow proportionally to $\xi_{h}$ until it remains much smaller than $\xi_{s}$. When $\xi_{h}$ exceeds $\xi_{s}$, both parameters have much weaker dependence on $\xi_{h}$. Moreover, the gap correction may decrease with increasing of $\xi_{h}$ in the vicinity of the magnetic transition. We also find that the crossover is unexpectedly broad: the standard scattering approximation becomes sufficient only when $\xi_{h}$ is substantially smaller than $\xi_{s}$.

We present a study of the magnetic-flux evolution in the magnetic superconductor RbEuFe4As4 performed using magneto-optical imaging and magnetization measurements during field cooling and warming and magnetic field cycling at temperatures above and below the magnetic transition point, T-m. The vortex patterns emerging at T less than or similar to T-m reveal that the Eu-spin subsystem serves as an internal pump of the magnetic flux while the superconducting critical current controls the delivery of magnetic-flux quanta into the bulk. The interplay of magnetic susceptibility amplifying the magnetic induction and vortex pinning attenuating the magnetic-flux entry results in a field- and temperature-dependent critical state that emulates a paramagnetic Meissner effect. The observed vortex dynamics corresponds to a nontrivial spatial current distribution and yields a self-consistent inhomogeneous enhancement of the sample magnetization.