Antiferromagnetic Superconductor Research Articles

Comment on "Observation of oscillatory magnetic order in the antiferromagnetic superconductor HoNi2Ba2C"

A Comment on the Letter by T. E. Grigereit {ital et} {ital al}., Phys. Rev. Lett. {bold 73}, 2756 (1994). The authors of the Letter offer a Reply.

Anisotropy of the upper critical field in URu 2Si 2 and FFLO state in antiferromagnetic superconductors

Experimental data of the upper critical field of the heavy-fermion superconductor URu 2Si 2 are analysed, in the clean limit, taking account of both orbital and Pauli limits. A discussion on the possible existence of a non-uniform superconducting state expected in clean Pauli-limited superconductors (FFLO state) follows. We also present calculations determining the influence of an AF ground state on the existence of such a non-uniform superconducting state.

Competition between magnetism and superconductivity in rare-earth nickel boride carbides.

The competition between magnetism and superconductivity was investigated for the recently discovered R${\mathrm{Ni}}_{2}$${\mathrm{B}}_{2}$C superconductors with the magnetic rare-earth elements R=Tm,Er,Ho,Dy. The systematic decrease of ${\mathit{T}}_{\mathit{c}}$, approximately scaled by the de Gennes factor, implies a very weak coupling between the rare-earth magnetic moments and the conduction electrons due to a small conduction-electron density at the rare-earth site. Associated with the antiferromagnetic order of the rare-earth moments, a pronounced dip structure in the upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) was observed; a similar structure has been seen in the previously known magnetic superconductors R${\mathrm{Rh}}_{4}$${\mathrm{B}}_{4}$ and R${\mathrm{Mo}}_{6}$${\mathrm{S}}_{8}$. For ${\mathrm{HoNi}}_{2}$${\mathrm{B}}_{2}$C, the pair-breaking associated with the magnetic transition is strong enough to bring about a resistive reentrant behavior even under zero field, an effect which, to our knowledge, has not previously been observed in antiferromagnetic superconductors.

Interplay of superconductivity and antiferromagnetism in SmRh4B4

A microscopic theory of interplay of superconductivity and antiferromagnetism in rare earth ternary systems is developed from first principles for less than half filledf atomic shells. Self consistent equations for the superconducting order parameter Δ and magnetic order parameter Γ, are derived using a Green’s function technique and equation of motion method. The theory is applied to explain the experimental results in the antiferromagnetic superconductor SmRh4B4. The present model explains true coexistence of superconductivity and antiferromagnetism and the suppression of superconductivity by antiferromagnetism. The behaviour of superconducting order parameter (Δ), magnetic order parameter (Γ), the specific heat, the density of states, free energy and critical field (H c) is also studied for the system SmRh4B4.

Transition temperature of proximity-effect antiferromagnetic superconductors sandwiches

The superconducting proximity effect between thin films of antiferromagnetic superconductor (AFS) is investigated using the McMillan tunneling model and the mean field model of AFS given by Nass, Levin and Grest. By assuming the nesting condition for the one-dimensional electron band, the spatial uniformity of the order parameter and the temperature independence of the staggered magnetic field, the transition temperature T c is derived and shown to cover the limiting cases of AFS-normal metal and AFS-superconductor systems. We find that the decrease of T c of the proximity bilayer is due to the proximity induced staggered field in the composite. The results of numerical calculations for T c for various cases are given and discussed.

Extended Emery models with antiferromagnetic and superconducting pairings

The mean-field (MF) study of electronic pairing in the CuO2 plane of high-temperature superconductors (HTSC) in the band approach is extended to Emery-like models including psigma -psigma hopping in addition to psigma -d hopping and a detailed picture of local interactions. Various types of correlated hopping interactions (CHI) are tested as factors that might induce superconductivity (SC). A classification of the order parameters (OP) for all types of pairing is presented for the upper band of such an extended three-band Emery model. Self-consistent equations for all OP are derived. The MF calculations of phase diagrams show that s-type SC may be induced by p-d or p-p CHI, but such systems (in the MF approach) do not exhibit antiferromagnetism (AF) in the undoped case. In contrast, models with large p-d CHI governed by p occupancy may exhibit both d-type SC and AF at small doping, and there is a range of doping where d-type AF and d-type SC may coexist.

Transport properties of antiferromagnetic superconductors

Transport properties of antiferromagnetic superconductors with T{sub N} < {Tc} were investigated. Detailed numerical, results are given for SmRh{sub 4}B{sub 4} by using the following model. The paramagnetic phase (T{sub N} < T < {Tc}) is described by using Abrikosov-Gorkov theory of magnetic ions in an ordinary superconductor. In the AF phase (T {le} T{sub N}), the effects of the molecular field H{sub Q}(T) and the elastic scattering of conduction electrons from spin fluctuations are included. Expressions for H{sub Q}(T) and the scattering rate from spin fluctuations were derived. The aim was to see if properties are enhanced or depressed by the AF ordering occurring below T{sub N}. It is found that whereas the electronic thermal conductivity K{sub s}, nuclear spin relaxation rate R{sub s}, and the longitudinal ultrasonic attenuation a{sub s}, are depressed by the AF ordering, the inverse of the magnetic penetration depth, ({lambda}(T)){sup {minus}1}, is enhanced below T{sub N}. The effect of all types of impurities is included in the study. Theoretical results for K{sub s} and ({lambda}(T)){sup {minus}1} agree with the experimental values for SmRh{sub 4}B{sub 4} (experimental data for other properties are not available in literature). In the above, T{sub N} is the Neel temperaturemore » and {Tc} is superconducting transition temperature.« less

On the possible refinement of the magnetic structure of the antiferromagnetic superconductor Tm2Fe3Si5

Abstract The thermodynamic symmetry analysis of the magnetic structure of the antiferromagnetic superconductor Tm 2 Fe 3 Si 5 was performed. We have shown, that the refined magnetic structure is principally noncollinear and exhibits weak antiferromagnetism.

Thermodynamic properties of antiferromagnetic superconductors containing nonmagnetic, magnetic, and spin-orbit impurities

Effects of all types of impurities (nonmagnetic, magnetic, and spin-orbit) on an antiferromagnetic superconductor (AFSC) have been investigated by studying the transition temperatureT c and the specific heat jump. We have assumed a one-dimensional electron band. The impurity scattering is treated within the self-consistent Born approximation. We find that: (a) the molecular fieldH Q and the magnetic impurities depress superconductivity of AFSC and their pair-breaking effect is additive; (b) the effect of spin-orbit impurities is the same as that of nonmagnetic impurities—these enhance superconductivity by screening the molecular field; and (c) in the extreme dirty limit, the AFSC is described in terms of an effective pair-breaking parameter given by 1/τeff=1/τ2+H Q 2 τ where 1/τ=1/τ1+2/3τso(1/τ1, 1/τ2, and 1/τso, respectively, are the scattering rates from nonmagnetic, magnetic and spin-orbit impurities).

The density of states and tunneling differential conductivity for an antiferromagnetic superconductor

The superconducting density of states ϱ S(ω) and the tunneling differential conductivity σ ns(V) for an antiferromagnetic superconductor (AFSC) are calculated both in the paramagnetic and in the antiferromagnetic phases. Our model for AFSC includes the effects of the molecular field, impurities and elastic scattering from spin-fluctuations. The experimental data on σ ns(V) for TbMO 6S 8 is understood reasonably well.

Influence of metamagnetic transition on the vortex phase in antiferromagnetic superconductors

Abstract The metamagnetic (or spin-flop) transition which is experimentally observed in certain antiferromagnetic superconductors results in the formation of magnetic domain structure if the demagnetization factor of the sample is nonzero. It is demonstrated that such structure leads to the modulation of vortex lattice density which is maximal in the domains of spin-flop phase. The structure of a vortex with a ferrimagnetic domain near its core is also investigated. The results of calculations show that under appropriate conditions the interplay between superconductivity and metamagnetism makes the formation of peculiar two-quanta vortices energetically favourable.

An analysis of the metamagnetic character of the mixed state in bct ErRh 4B 4

A theoretical analysis of a magnetic field induced transition in the mixed state of an antiferromagnetic superconductors is presented. The electromagnetic properties of the superconducting mixed state are calculated within the framework of the boson method while the magnetic properties of the rare earth ions are represented by means of a Landau expansion. The form of the magnetic free energy and the Landau coefficients are chosen to model the metamagnetic behaviour observed in bct ErRh 4B 4. Phase diagrams and magnetisation curves are presented for various choices of parameters and a comparison with the recent observations on bct ErRh 4B 4 is made. The analysis of the antiferromagnetic phase is extended to include the possible formation of a ferrimagnetic domain around the vortex core and its effect is discussed.

Theory of the electron-spin susceptibility in antiferromagnetic superconductors

A theory of the longitudinal electron spin susceptibility ${\ensuremath{\chi}}_{s}$ for an antiferromagnetic superconductor (AFS) has been given. For the AFS, we have assumed a homogeneous superconducting order parameter and a one-dimensional electron band that satisfies the nesting condition ${\ensuremath{\varepsilon}}_{\mathrm{k}}$=-${\ensuremath{\varepsilon}}_{\mathrm{k}+\mathrm{Q}}$, where Q is the wave vector characterizing the antiferromagnetic (AF) order. First we have studied the dependence of ${\ensuremath{\chi}}_{s}$ on the scattering rates for the scattering of conduction electrons from the nonmagnetic, spin-orbit, and magnetic impurities by regarding ${H}_{Q}$ as a parameter and neglecting the spin-fluctuation effects (${H}_{Q}$ is the AF field). The effect of impurities is found to be significant. Then we have investigated the temperature dependence of ${\ensuremath{\chi}}_{s}$ by taking ${T}_{N}$${T}_{c}$ and by including the spin-fluctuation effects and the temperature dependence of the AF field (${T}_{N}$ is the AF ordering temperature and ${T}_{c}$ is the superconducting transition temperature). The aim has been to see if ${\ensuremath{\chi}}_{s}$ is enhanced or depressed by the AF ordering occurring below ${T}_{N}$. We find that (1) ${\ensuremath{\chi}}_{s}$ increases with the increase in scattering rate from spin-orbit impurities both above and below ${T}_{N}$; (2) keeping other parameters fixed, the enhancement or depression of ${\ensuremath{\chi}}_{s}$ below ${T}_{N}$ depends on ${H}_{Q}$(0)---there is enhancement (depression) when ${H}_{Q}$(0) is larger (smaller) [${H}_{Q}$(0) is the zero-temperature value of ${H}_{Q}$]; (3) nonmagnetic impurities have a dramatic effect on ${\ensuremath{\chi}}_{s}$ in the AF phase. For cleaner (dirtier) superconductors, ${\ensuremath{\chi}}_{s}$ is enhanced (depressed) below ${T}_{N}$; (4) in ${\mathrm{SmRh}}_{4}$${\mathrm{B}}_{4}$, one expects a depression in ${\ensuremath{\chi}}_{s}$ by the AF ordering.

The order parameter and the Josephson current for an antiferromagnetic superconductor

The superconducting order parameter Δ( T) and the maximum Josephson tunneling current J s ( T) for an antiferromagnetic superconductor (AFS) are calculated both in the paramagnetic and the antiferromagnetic phases. The effects of the molecular field, impurities and the elastic scattering from spin-fluctuations have been included. The enhancement in J s ( T) and Δ( T) by the antiferromagnetic (AF) ordering observed in SmRh 4B 4 by Vaglio et al. [1] can be understood with a reasonable choice of parameters. We also predict depression in J s ( T) and Δ( T) by AF ordering in some materials.

Penetration depths of antiferromagnetic superconductors SmRh 4B 4 and NdRh 4B 4

We have investigated the surface impedance penetration depth, ⋀, both experimentally and theoretically of two antiferromagnetic superconductors, SmRh 4B 4 and NdRh 4B 4 which exhibit quite different temperature dependencies of the upper critical field arising from two different mechanisms of the modification of the superconductivity by the presence of the antiferromagnetism: the antiferromagnetic molecular field effect and the spin-fluctuation effect. It has been shown the importance of the fluctuation effect for SmRh 4B 4 (⋀ −1 increases more rapidly below T N ) in contrast to NdRh 4B 4 where the molecular field effect is more significant (⋀ −1 decreases below T N ).

High-field transitions in URu 2Si 2 Observed by magnetoresistivity and magnetization experiments

High-field magnetoresistivity and magnetization experiments have been performed on a monocrystalline sample of the antiferromagnetic superconductor URu 2Si 2, for a field direction along the tetragonal axis. At a temperature of 1.5K three sharp transitions are observed at fields of 35.8, 37.3 and 39.4T, suggesting a complex magnetization process. Large jumps in the magnetoresistance indicate that the magnetization process involves gap-like changes of the Fermi surface.

The critical curve in an antiferromagnetic superconductor with nonmagnetic impurities

The critical factor curve of a transition of the second kind in an antiferromagnetic superconductor (AFS) with nonmagnetic impurities has been studied. The AFS is described by using the mean-field model given by Nass, Levin, and Grest and assuming a one-dimensional electron band. They find that the points on the critical curve satisfy the thermodynamic stability condition for 0 less than or equal to ..cap alpha../sub 1 tau//..delta../sub 0/ less than or equal to 5.04 and 0.49 less than or equal to H/sub Q tau//..delta../sub 0/ less than or equal to 1.64. Here ..cap alpha../sub 1/ is the inverse lifetime of a conduction electron for nonmagnetic impurity scattering, H/sub Q/ is the antiferromagnetic molecular field, ..delta../sub 0/ is the zero-temperature order parameter of a superconductor in the absence of H/sub Q/ and impurities. Further, ..cap alpha../sub 1 tau/ and H/sub Q tau/ denote the values of these quantities for points on the critical curve. For ..cap alpha../sub 1 tau//..delta../sub 0/ > 5.04 and H/sub Q tau//..delta../sub 0/ > 1.64, the phase transition from the superconducting to the normal state is always of the second kind. Some thermal properties of the system near the critical curve have also been investigated andmore » they find that these depends dramatically on the impurity concentration.« less

Theoretical studies on magnetic superconductors

The discovery of magnetic superconductors has posed the problem of the coexistence of two kinds of orders (magnetic and superconducting) in some temperature intervals in these systems. New microscopic mechanisms developed by us to explain the coexistence and reentrant behaviour are reported. The mechanism for antiferromagnetic superconductors which shows enhancement of superconductivity below the magnetic transition is found relevant for rare-earth systems having less than half-filled f-atomic shells. The theory will be compared with the experimental results of SmRh4B4 system. A phenomenological treatment based on a generalized Ginzburg-Landau approach will also be presented to explain the anomalous behaviour of the second critical field in some antiferromagnetic superconductors. These magnetic superconductors provide two kinds of Bose fields, namely, phonons and magnons which interact with each other and also with the conduction electrons. Theoretical studies of the effects of the excitations of these modes on superconducting pairing and magnetic ordering in these systems will be discussed.

Penetration depths of antiferromagnetic superconductors: NdRh4B4 and SmR4B4 (abstract)

We have investigated two antiferromagnetic superconductors, NdRh4B4 and SmRh4B4, which exhibit two quite different temperature dependencies of the upper critical field arising from two different mechanisms1 of the modification of superconductivity by the presence of antiferromagnetism. In order to further study these mechanisms, we have investigated the electromagnetic properties and have measured the penetration depths λ(T) of NdRh4B4 (Tc=5.45 K, TN1=1.31 K, and TN2=0.89 K) and SmRh4 B4 (Tc=2.2 K, TN=0.9 K), employing a tunnel diode oscillator at ∼30 MHz. Above the antiferromagnetic transition temperature (TN1 for NdRh4B4 and TN for SmRh4B4), the temperature dependencies of the penetration depths of these two superconductors fit the BCS theory well. We have found λ(0)=2170 and 980 Å for NdRh4B4 and SmRh4B4, respectively. However, as temperature is lowered, λ(T) for NdRh4B4 increases near TN1 and then saturates near TN2, presumably owing to the molecular field effect.2 The spin-fluctuation effects,2 expected to be significant near TN2 for NdRh4B4 and TN for SmRh4B4, have not been observed within our experimental accuracy.

Neutron scattering studies of heavy fermion systems (invited) (abstract)

We review our results of neutron scattering experiments on the heavy electron systems UPt3, U2Zn17, and URu2Si2. In the superconductor UPt3 we have established the existence of spin fluctuations on an energy scale of 0.5 meV and correlated in the same structure as the antiferromagnetic order in the Th- and Pd-doped systems. In the antiferromagnet U2Zn17 overdamped antiferromagnetically correlated excitations dominate the excitation spectrum above and below TN. However, in an applied field the resonances are induced in the transverse response. In the antiferromagnetic superconductor URu2Si2 the excitations in the ordered phases are propagating, longitudinal and similar to observations in singlet rare-earth ground-state systems. However, the ordered moment of 0.03μB is anomalously small and the transverse response is overdamped. We discuss a simple model of exchange coupled overdamped responses which gives a consistent description of the diffusive spin-fluctuations in several heavy electron systems.