Antiferromagnetic Superconductor Research Articles

Theoretical valley-polarized subgap transport and intravalley pairing states in a silicene-based antiferromagnet–superconductor junction

We theoretically study the valley-polarized subgap transport and intravalley pairing states in silicene-based antiferromagnet/superconductor (AF/SC) junctions. It is found that in the absence of an electric field, the antiferromagnetic order induced in silicene can give rise to valley-polarized states that strongly affect the subgap conductance. With the increasing antiferromagnetic exchange field, the gap-edge Andreev-resonant peak is replaced by broadened features for the homo-SC model whereas by a sharp conductance dip for the bulk-SC one. This significant difference arises from the intravalley Andreev reflection caused by the valley-mixing scattering in the bulk-SC model, which can be enhanced by the antiferromagnetic order. Particularly, this intravalley pairing process can be switched on or off by adjusting the spin polarization through the electric field applied in the AF region. Our findings not only pave a new road to employ antiferromagnetic materials in valleytronics, but also facilitate the verification and detection of potential intravalley pairing state and valley polarization in silicene.

New Type of “Dark” States in the Spectrum of Radiative Magnon Polarons

The appearance of new types of bound (“dark”) states in the continuous spectrum of shear phonons of the radiation field of leaky magnon polarons in an acoustically open “easy-axis antiferromagnet–superconductor” sandwich structure can be determined by the relative orientation of the plane of incidence of a transverse elastic wave and the equilibrium antiferromagnetism vector in the interface plane between the media. The effect is based on the elastic dipole mechanism of the formation of magneto-acoustic birefringence with a change of the refraction cavity induced by the symmetric or antisymmetric Dzyaloshinskii interaction.

Electrical control of crossed Andreev reflection and spin-valley switch in antiferromagnet/superconductor junctions

We study the subgap transport through the antiferromagnet/superconductor (AF/S) and antiferromagnet/superconductor/antiferromagnet (AF/S/AF) junctions controlled by electric field in a generic buckled honeycomb system, such as silicene, germanene, and stanene. In the presence of electric field and antiferromagnetic exchange field, the spin-valley polarized half-metallic phase can be achieved in the honeycomb system due to the spin-orbit coupling, which affords an opportunity to generate the pure crossed Andreev reflection (CAR). It is found that the pure CAR can be generated without local Andreev reflection (AR) and elastic cotunneling (EC) over a wide range of electric field. A spin-valley switch effect can be realized between the pure CAR and the pure EC by adjusting the electric field. The properties of AR process and CAR process strongly depend on the spin-valley polarized states. Our results suggest that the device can implement an electrical measurement of the CAR process and spin-valley switch.

Electrical and thermal transport in antiferromagnet-superconductor junctions

We demonstrate that antiferromagnet-superconductor (AF-S) junctions show qualitatively different transport properties than normal metal-superconductor (N--S) and ferromagnet-superconductor (F-S) junctions. We attribute these transport features to presence of two new scattering processes in AF--S junctions, i.e., specular reflection of holes and retroreflection of electrons. Using the Blonder-Tinkham-Klapwijk formalism, we find that the electrical and thermal conductance depend nontrivially on antiferromagnetic exchange strength. Furthermore, we show that the interplay between the N\'eel vector direction and the interfacial Rashba spin-orbit coupling leads to a large anisotropic magnetoresistance. The unusual transport properties make AF--S interfaces unique among the traditional condensed-matter-system-based superconducting junctions.

Unfrustrating the t-J Model: Exact d-wave BCS Superconductivity in the t'-J_z-V Model

The t-J model is believed to be a minimal model that may be capable of describing the low-energy physics of the cuprate superconductors. However, although the t-J model is simple in appearance, obtaining a detailed understanding of its phase diagram has proved to be challenging. We are therefore motivated to study modifications to the t-J model such that its phase diagram and mechanism for d-wave superconductivity can be understood analytically without making uncontrolled approximations. The modified model we consider is a t’-J_zJz-V model on a square lattice, which has a second-nearest-neighbor hopping t’ (instead of a nearest-neighbor hopping t, an Ising (instead of Heisenberg) antiferromagnetic coupling J_zJz, and a nearest-neighbor repulsion V. In a certain strongly interacting limit, the ground state is an antiferromagnetic superconductor that can be described exactly by a Hamiltonian where the only interaction is a nearest-neighbor attraction. BCS theory can then be applied with arbitrary analytical control, from which nodeless d-wave or s-wave superconductivity can result.

Anisotropy of the Hall Effect in a Quasi-Two-Dimensional Electron-Doped Nd2 – xCexCuO4 + δ Superconductor

Abstract—The temperature dependences of the Hall effect of an electron-doped Nd2 – xCexCuO4 + δ superconductor have been studied at the antiferromagnet–superconductor quantum phase transition boundary (0.135 ≤ x ≤ 0.15) in conducting CuO2 planes and in the direction perpendicular to the CuO2 plane. The hall coefficient between the conducting planes has been found be two orders higher than that in the conducting planes over entire temperature range, which is due to the incoherent character of the charge carrier transfer in the direction of axis c.

Symmetry-protected line nodes and Majorana flat bands in nodal crystalline superconductors

Line nodes in the superconducting gap are known to be a source of Majorana flat bands (MFBs) in time-reversal-invariant superconductors (SCs). Here, we extend this relation to all symmetry-protected line nodes where an additional constraint arising from a symmetry of the crystal destabilizes or hides the existence of MFBs. By establishing a one-to-one correspondence between group theoretical and topological classifications, we are able to classify the possible line-node-induced MFBs, including cases with (magnetic) non-symmorphic space groups. Our theoretical analysis reveals a new type of MFB, i.e., MFBs in antiferromagnetic SCs.

Анизотропия эффекта Холла в квазидвумерном электронно-легированном сверхпроводнике Nd-=SUB=-2-x-=/SUB=-Ce-=SUB=-x-=/SUB=-CuO-=SUB=-4+delta-=/SUB=-

Abstract —The temperature dependences of the Hall effect of an electron-doped Nd_2 – _ x Ce_ x CuO_4 + δ superconductor have been studied at the antiferromagnet–superconductor quantum phase transition boundary (0.135 ≤ x ≤ 0.15) in conducting CuO_2 planes and in the direction perpendicular to the CuO_2 plane. The hall coefficient between the conducting planes has been found be two orders higher than that in the conducting planes over entire temperature range, which is due to the incoherent character of the charge carrier transfer in the direction of axis c .

Correlation between the Hall Resistance and Magnetoresistance in the Mixed State of an Nd2 − xCe x CuO4 + δ Electronic Superconductor

The Hall resistance and the magnetoresistance in the mixed state of the Nd2 − xCe x CuO4 + δ quasi-two-dimensional system near the antiferromagnetic-superconductor (AF-SC) phase transition have been measured at doping levels x = 0.14 and 0.15, and a correlation has been established. This correlation can be analyzed using the following power relationship: ρ xy (B) ~ [ρ xx (B)]β. It was found that index β varied from 0.94 ± 0.03 in the region of AF and SC coexistence (x = 0.14) to 0.6 ± 0.1 in the SC region with the maximum critical temperature (x = 0.15) at low temperatures and weak magnetic fields. This reduction suggests that the symmetry of carrier pairing changes at the boundary of the transition from the phase of antiferromagnetic ordering and spin density waves to the superconducting phase in the presence of antiferromagnetic spin fluctuations.

Symmetry-Enforced Line Nodes in Unconventional Superconductors.

We classify line nodes in superconductors with strong spin-orbit interactions and time-reversal symmetry, where the latter may include nonprimitive translations in the magnetic Brillouin zone to account for coexistence with antiferromagnetic order. We find four possible combinations of irreducible representations of the order parameter on high-symmetry planes, two of which allow for line nodes in pseudospin-triplet pairs and two that exclude conventional fully gapped pseudospin-singlet pairs. We show that the former can only be realized in the presence of band-sticking degeneracies, and we verify their topological stability using arguments based on Clifford algebra extensions. Our classification exhausts all possible symmetry protected line nodes in the presence of spin-orbit coupling and a (generalized) time-reversal symmetry. Implications for existing nonsymmorphic and antiferromagnetic superconductors are discussed.

Symmetry-Protected Line Nodes in Non-symmorphic Magnetic Space Groups: Applications to UCoGe and UPd2Al3

We present the group-thoretical classification of gap functions in superconductors coexisting with some magnetic order in non-symmorphic magnetic space groups. Based on the weak-coupling BCS theory, we show that UCoGe-type ferromagnetic superconductors must have horizontal line nodes on either $k_z=0$ or $\pm\pi/c$ plane. Moreover, it is likely that additional Weyl point nodes exist at the axial point. On the other hand, in UPd$_2$Al$_3$-type antiferromagnetic superconductors, gap functions with $A_g$ symmetry possess horizontal line nodes in antiferromagnetic Brillouin zone boundary perpendicular to $c$-axis. In other words, the conventional fully-gapped $s$-wave superconductivity is forbidden in this type of antiferromagnetic superconductors, irrelevant to the pairing mechanism, as long as the Fermi surface crosses a zone boundary. UCoGe and UPd$_2$Al$_3$ are candidates for unconventional superconductors possessing hidden symmetry-protected line nodes, peculiar to non-symmorphic magnetic space groups.

Nonuniform superconducting state in an antiferromagnetic superconductor

We consider the superconducting state in a clean crystal with antiferromagnetic (AF) structure of localized magnetic moments taking into account the exchange interaction between magnetic moments and conduction electrons. We assume that the localized moments order at the Neel temperature ${T}_{N}$ due to the RKKY interaction predominantly. In such crystals, the periodic exchange field acting on conducting electrons results in the formation of an insulating gap on the Fermi surface for electrons moving in directions that depend on the orientation of the wave vector of AF ordering. We assume a scenario in which the Cooper pairing occurs in the open parts of the Fermi surface at the temperature ${T}_{c}$. We show that at high amplitudes of exchange field ${h}_{e}g{T}_{c}/{\ensuremath{\mu}}_{B}$, the structure of superconducting state just below the temperature ${T}_{c}$ depends on the relation between ${T}_{c}$ and ${T}_{N}$. At low ratio ${T}_{c}/{T}_{N}$, a nonuniform superconducting state, like the Fulde-Farrell-Larkin-Ovchinnikov high-field phase, should exist, while at bigger ratio superconducting order parameter is uniform. The nonuniform structure of superconducting state may be probed by tunneling measurements.

Interplay Between Superconductivity and Antiferromagnetism in HoNi2B2C

A microscopic theory of interplay between superconductivity and antiferromagnetism in rare-earth nickel boride, HoNi2B2C is developed from first principles. Self-consistent equations for the superconducting order parameter Δ and magnetic order parameter Γ are derived using a Green’s function technique and an equation of motion method. The theory is applied to explain the experimental results in the antiferromagnetic superconductor HoNi2B2C. The present model explains the true coexistence of superconductivity and antiferromagnetism in this system. The behavior of the superconducting order parameter (Δ), the magnetic order parameter (Γ), the specific heat, the density of states, the free energy and critical field (Hc) is also studied for the system HoNi2B2C. Distinct features of the coexistence region are discussed. There is the convincing evidence that the theory is fully compatible with the key experiments.

Quantum creep in layered antiferromagnetic superconductor

In the mixed state of layered superconductor, the antiferromagnetic order of magnetic ions can create the spin-flop domains along the phase cores of the Josephson vortices, and this property impact upon the creep rate in the antiferromagnetic superconductor. The activation of the creep at constant temperature can be either thermal or quantum, depending on the intensity, or direction of the applied magnetic field. It is also shown that the action, and hence the activation energy, is rendered temperature dependent, when the damping and inertial mass of the vortex are included, so that the quantum tunnelling rate becomes temperature dependent below the crossover temperature.

Disorder Effects on Competition between Antiferromagnetism and Superconductivity in Cuprate Superconductors through the Enhancement in Charge Susceptibility

The coexistence state of antiferromagnetism (AF) and superconductivity (SC) has been observed in five-layered cuprates. However, this coexistence state disappears, and the AF phase and SC phase lose contact in the doping phase diagram toward double- and single-layered cuprates. We investigate the mechanism of the disappearance of the coexistence of AF and SC in disordered cuprate superconductors in order to understand these doping phase diagrams. In single- and double-layered cuprates, electrons on the CuO$_2$ plane experience the disorder effect through inhomogeneity in the charge reservoir layer. These impurity potentials can be effectively enhanced toward the underdoped region by the effect of many-body corrections that involve an increase in charge susceptibility. As a result, strong disorder effects are expected particularly in the competing regions of AF and SC, where the coexistence phase of AF and SC is extremely suppressed. We show the validity of this suppression mechanism by considering the Aslamazov-Larkin-type vertex correction to the effective impurity potential in the effective mean-field phase diagram.

Nanoscale Layering of Antiferromagnetic and Superconducting Phases inRb2Fe4Se5Single Crystals

We studied phase separation in the single-crystalline antiferromagnetic superconductor Rb(2)Fe(4)Se(5) (RFS) using a combination of scattering-type scanning near-field optical microscopy and low-energy muon spin rotation (LE-μSR). We demonstrate that the antiferromagnetic and superconducting phases segregate into nanometer-thick layers perpendicular to the iron-selenide planes, while the characteristic in-plane size of the metallic domains reaches 10 μm. By means of LE-μSR we further show that in a 40-nm thick surface layer the ordered antiferromagnetic moment is drastically reduced, while the volume fraction of the paramagnetic phase is significantly enhanced over its bulk value. Self-organization into a quasiregular heterostructure indicates an intimate connection between the modulated superconducting and antiferromagnetic phases.

Iron based superconductors: Pnictides versus chalcogenides

We present a brief review of the present day situation with studies of high-temperature superconductivity in iron pnictides and chalcogenides. Recent discovery of superconductivity with Tc>30K in AxFe2−x/2Se2 (A=K, Cs, Tl, etc) represents the major new step in the development of new concepts in the physics of Fe-based high-temperature superconductors. We compare LDA and ARPES data on the band structure and Fermi surfaces of novel superconductors and those of the previously studied FeAs superconductors, especially isostructural 122-superconductors like BaFe2As2. It appears that electronic structure of new superconductors is rather different from that of FeAs 122-systems. In particular, no nesting properties of electron and hole-like Fermi surfaces is observed, casting doubts on most popular theoretical schemes of Cooper pairing for these systems. Doping of novel materials is extremely important as a number of topological transitions of Fermi surface near the Γ point in the Brillouin zone are observed for different doping levels. The discovery of Fe vacancies ordering and antiferromagnetic (AFM) ordering at pretty high temperatures (TN>500K), much exceeding superconducting Tc makes these systems unique antiferromagnetic superconductors with highest TN observed up to now. This poses very difficult problems for theoretical understanding of superconductivity. We discuss the role of both vacancies and AFM ordering in transformations of band structure and Fermi surfaces, as well as their importance for superconductivity. In particular, we show that system remains metallic with unfolded Fermi surfaces quite similar to that in paramagnetic state. Superconducting transition temperature Tc of new superconductors is discussed within the general picture of superconductivity in multiple band systems. It is demonstrated that both in FeAs-superconductors and in new FeSe-systems the value of Tc correlates with the value of the total density of states (DOSs) at the Fermi level.

Effect of Fe substitution on magnetocaloric effect in borocarbide superconductor Dy(Ni1-xFex)2B2C

Abstract The magnetic properties and magnetocaloric effect (MCE) in antiferromagnetic superconductor Dy(Ni1- x Fe x )2B2C ( x = 0, 0.1, and 0.2) has been studied. The magnetic transition temperature T M as well as the temperature at the maximum of magnetic entropy change (-Δ S M max ) slightly shift to low temperature with increasing x . An inverse MCE was observed, which is attributed to the nature of antiferromagnetic state under low magnetic field and at low temperatures for the present Dy(Ni1- x Fe x )2B2C compounds. A normal large MCE was observed under higher magnetic field changes, which is related to a field-induced first order meatmagnetic transition from antiferromagnetic to ferromagnetic state. Under a magnetic field change of 7 T, the values of -ΔSM max reach 22.1, 16.6, and 14.8 J Kg -1 K -1 for x = 0, 0.1, and 0.2, respectively. The corresponding values of relative cooling power are 464, 332 and 295 J/kg. The observed large magnetic entropy is related to a field-induced first order metamagnetic transition from antiferromagnetic to ferromagnetic state.

ChemInform Abstract: Studies on Molecular Conductors: From Organic Semiconductors to Molecular Metals and Superconductors

AbstractReview: 130 refs.

COOPER PAIR BREAKING

An overview is given of a number of pair-breaking interactions in superconductors. They have in common that they violate a symmetry of the pair state. In most cases pairs are formed from time reversed single-particle states, a noticeable exception being antiferromagnetic superconductors. When time reversibility is broken by an interaction acting on the electrons, the time evolution of the time-reversal operator plays an important role. Depending on whether it is nonergodic or ergodic, we deal with pair weakening or pair breaking. Numerous different interactions are analyzed and discussed. Unifying features of different pair-breaking cases are pointed out. Special attention is paid to the Zeeman effect and to scattering centers with low-energy excitations. The Kondo effect and crystalline field split rare-earth ions belong in that category. Modifications caused by strongly anisotropic pair states are pointed out. There is strong evidence that in some cases intra-atomic excitations lead to pair formation rather than pair breaking for which an explanation is provided.