Coexistence Of Superconductivity Research Articles

Coexistence of Ferromagnetism and Superconductivity in Noncentrosymmetric Materials with Cubic Symmetry

This is a model study for the emergence of superconductivity in ferromagnetically ordered phases of cubic materials whose crystal structure lacks inversion symmetry. A Ginzburg–Landau-type theory is used to find the ferromagnetic state and to determine the coupling of magnetic order to superconductivity. It is found that noncentrosymmetricity evokes a helical magnetic phase. If the wavelength of the magnetic order is long enough, it gives rise to modulations of the order parameter of superconductivity, both in modulus and complex phase. At magnetic domain walls the nucleation of superconductivity is found to be suppressed as compared to the interior of ferromagnetic domains.

Ab Initio Probing of the Magnetic and Electronic Properties of ThCr2Si2-Like Charge-Balanced KFeAgTe2

Based on density functional theory band structure calculations, we address the magnetic and electronic properties of KFeAgTe2 system. This phase belongs to the newest family of ThCr2Si2-type ternary iron-chalcogenide materials AxFe2−yCh2 (A= alkali metals or Tl, Ch= Se or S, discovered in 2010) which have aroused recently a tremendous interest and triggered intense studies owing to intriguing coexistence of metal vacancy ordering, antiferromagnetism, and superconductivity. But, unlike AxFe2−yCh2, KFeAgTe2 is a Te-containing ThCr2Si2-type phase, includes a large amount of the noble metal and is free of vacancies. Here, by means of first-principles FLAPW-GGA calculations, the spin ordering of the magnetic ground state, total and partial densities of states for KFeAgTe2 were obtained for the first time and analyzed in comparison with the related AxFe2−yCh2 systems.

ChemInform Abstract: Properties of Ferromagnetic Superconductors

AbstractReview: 66 refs.

Analysis of local chemical and structural inhomogeneities in FeySe1−xTex single crystals

Co-existence of superconductivity and magnetism has been reported in certain regions of the Fe(Se,Te) phase diagram. Here, we address the key question of whether these different properties occur simultaneously within a homogeneous crystal or whether a “two-phase” description is more appropriate. Extensive microstructural analysis suggests that a Te-rich sample exhibiting both bulk superconductivity and magnetic order contains significant chemical and structural inhomogeneity, suggesting that the “two-phase” explanation may be valid in this sample. The high-resolution electron backscatter diffraction (HR-EBSD) technique has been used to map local changes in lattice parameter very precisely in these compounds.

Interplay between magnetism and superconductivity in EuFe2−xCoxAs2studied by57Fe and151Eu Mössbauer spectroscopy

The compound EuFe${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$ was investigated by means of ${}^{57}$Fe and ${}^{151}$Eu M\"ossbauer spectroscopy versus temperature (4.2--300 K) for $x$ = 0 (parent), $x$ = 0.34--0.39 (superconductor), and $x$ = 0.58 (overdoped). It was found that the spin density wave (SDW) is suppressed by Co substitution; however, it survives in the region of superconductivity, but iron spectra exhibit some nonmagnetic components in the superconducting region. Europium orders magnetically, regardless of the cobalt concentration, with the spin reorientation from the $a$-axis in the parent compound toward the $c$-axis with increasing replacement of iron by cobalt. The reorientation takes place close to the $a$-$c$ plane. Some trivalent europium appears in EuFe${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$ versus substitution due to the chemical pressure induced by Co atoms, and it experiences some transferred hyperfine field from Eu${}^{2+}$. Iron experiences some transferred field due to the europium ordering for substituted samples in the SDW and nonmagnetic state both, while the transferred field is undetectable in the parent compound. Superconductivity coexists with the 4f-europium magnetic order within the same volume. It seems that superconductivity has some filamentary character in EuFe${}_{2\ensuremath{-}x}$Co${}_{x}$As${}_{2}$, and it is confined to the nonmagnetic component seen by the iron M\"ossbauer spectroscopy.

Coexistence of superconductivity and magnetism in KxFe2−ySe2−zSz(z=0,0.4)

Single crystals K${}_{\mathrm{x}}$Fe${}_{2\ensuremath{-}y}$Se${}_{2\ensuremath{-}z}$S${}_{\mathrm{z}}$ ($z$ $=$ 0, 0.4) are successfully synthesized by self-flux method with the superconducting transition temperatures ${T}_{c}$${}^{\mathrm{onset}}$ of 31.3 K and 32.8 K, respectively. In contrast to external pressure effect on superconductivity, the substitution of S for Se does not suppress ${T}_{c}$, which suggests that chemical doping may mainly modulate the anion height from the Fe layer rather than compressing interlayer distance. In electron spin resonance (ESR) measurements, a resonance signal arising from the paramagnetic (PM) Fe ions is detected at room temperature for both K${}_{\mathrm{x}}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ and K${}_{\mathrm{x}}$Fe${}_{2\ensuremath{-}y}$Se${}_{1.6}$S${}_{0.4}$ crystals. Upon cooling, the intensity of the spectrum weakens gradually and displays abrupt decrease below 140 K. The resonance signal disappears around 60 K (\ensuremath{\sim}2 ${T}_{c}$), corresponding to the saturation temperature of antiferromagnetic (AFM) order revealed by recent neutron-diffraction study. The evolution of ESR spectrum is discussed by correlating with recent neutron-diffraction and Raman-scattering results, in support of the coexistence of superconductivity and AFM order in K${}_{\mathrm{x}}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$. Moreover, the variation of resonance field with decreasing temperature for K${}_{\mathrm{x}}$Fe${}_{2\ensuremath{-}y}$Se${}_{1.6}$S${}_{0.4}$ implies that the doping of S induces local weak ferromagnetism in the parent AFM background.

Anisotropic magnetism, superconductivity, and the phase diagram of Rb1−xFe2−ySe2

We report the crystal growth and structural, magnetic, conductivity, and specific heat investigations of Rb(1-x)Fe(2-y)Se2 single crystals with varying stoichiometry prepared by self-flux and Bridgman methods. The system exhibits a strongly anisotropic antiferromagnetic behavior below 400 K. For 1.53 <2-y< 1.6 superconductivity is found, whereas for Fe concentrations 2-y< 1.5 and 2-y> 1.6 insulating and semiconducting behavior is observed, respectively. The sharpest transition to the superconducting state and the highest transition temperature T_c of 32.4 K is found for compositions close to Rb2Fe4Se5. A critical current density j_c of 1.6*10^4 A/cm^2 (at 2 K) is obtained in the superconducting samples with the highest diamagnetic response. In these samples additional intense x-ray reflexes appear, which are incompatible with the tetragonal symmetry. Upper critical fields H_{c2} of ~250 kOe for the in-plane and 630 kOe for the inter-plane configurations are estimated from resistivity in magnetic fields parallel and perpendicular to the c-axis, respectively. In the non-superconducting samples with the Fe concentration below 1.45 both specific heat and susceptibility revealed an anomaly at 220 K which is not related to antiferromagnetic or structural transformations. Comparison with the magnetic behavior of non-superconducting samples provides evidence for the coexistence of superconductivity and static antiferromagnetic order.

Coexistence of Superconductivity and Magnetism

In the age of technology, with smaller and smaller electronic components being used in a growing number of applications, one pertinent application of mathematics and physics is the study of superconductivity. Superconductive materials are capable of conducting electricity without any resistance and were first discovered by Kamerlingh Onnes in 1911 in a compound He at 4.2 K in what was to prove to be one of the most significant scientific breakthroughs of the 20th Century. Superconductivity and Magnetism are mutually exclusive to each other but the coexistence of both the phenomena leads to very interesting research of superconductivity at room temperature. In this paper, we have studied the effect of ferromagnetic and antiferromagnetic interactions on the superconducting transition temperature.

Coexistence of antiferromagnetism and superconductivity withint-Jmodel with strong correlations and nonzero spin polarization

The coexistence of antiferromagnetism with superconductivity is studied theoretically within the t-J model with the Zeeman term included. The strong electron correlations are accounted for by means of the extended Gutzwiller projection method within a statistically-consistent approach proposed recently. The phase diagram on the band filling - magnetic field plane is shown, and subsequently the system properties are analyzed for the fixed band filling n=0.97. In this regime, the results reflect principal qualitative features observed recently in selected heavy fermion systems. Namely, (i) with the increasing magnetic field the system evolves from coexisting antiferromagnetic-superconducting phase, through antiferromagnetic phase, towards polarized paramagnetic state, and (ii) the onset of superconducting order suppresses partly the staggered moment. The superconducting gap has both the spin-singlet and the staggered-triplet components, a direct consequence of a coexistence of the superconducting state with antiferromagnetism.

Coexistence of superconductivity and incommensurate magnetic order

The influence of incommensurate spin-density waves (SDW) on superconductivity in unconventional superconductors is studied by means of the Bogolubov-de Gennes (BdG) equations. Exploiting translational symmetries of a magnetically ordered two-dimensional system, we propose an approach that allows to solve the BdG equations on much larger clusters than it is usually possible for inhomogeneous systems. Applying this approach, we demonstrate that the presence of incommensurate spin-density waves induces real-space inhomogeneity of the superconducting order parameter even in the absence of external magnetic field. In this case, a homogeneous order parameter of the Bardeen-Cooper-Schrieffer-type superconducting state is slightly modulated, or equivalently, a small fraction of the charge carriers form Cooper pairs with nonzero total momentum. However, when a sufficiently strong magnetic field is applied, the homogeneous component of the order parameter is suppressed and the system transits to the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, where the order parameter oscillates changing sign. We show that for $s$-wave pairing, the presence of external magnetic field diminishes the destructive influence of the SDW order on superconductivity. A simple explanation of this effect is also proposed.

Coexistence of Dirac-cone states and superconductivity in iron pnictide Ba(Fe1−xRuxAs)2

The Ru doping effect on the Dirac-cone states is investigated in iron pnictide superconductors Ba(Fe${}_{1\ensuremath{-}x}$Ru${}_{x}$As)${}_{2}$ using transverse magnetoresistance (${R}_{\mathrm{M}}$) measurements as a function of temperature. The linear development of ${R}_{\mathrm{M}}$ against magnetic field $B$ is observed for $x=0$--0.244 at low temperatures below the antiferromagnetic transition. The $B$-linear ${R}_{\mathrm{M}}$ is interpreted in terms of the quantum limit of the Dirac-cone states by using the model proposed by Abrikosov. An intriguing evidence is shown that the Dirac-cone state persists on the electronic phase diagram where antiferromagnetism and superconductivity coexist.

Direct imaging of the coexistence of ferromagnetism and superconductivity at the LaAlO3/SrTiO3 interface

LaAlO3 and SrTiO3 are insulating, nonmagnetic oxides, yet the interface between them exhibits a two-dimensional electron system with high electron mobility,1 superconductivity at low temperatures,2-6 and electric-field-tuned metal-insulator and superconductorinsulator phase transitions.3,6-8 Bulk magnetization and magnetoresistance measurements also suggest some form of magnetism depending on preparation conditions5,9-11 and suggest a tendency towards nanoscale electronic phase separation.10 Here we use local imaging of the magnetization and magnetic susceptibility to directly observe a landscape of ferromagnetism, paramagnetism, and superconductivity. We find submicron patches of ferromagnetism in a uniform background of paramagnetism, with a nonuniform, weak diamagnetic superconducting susceptibility at low temperature. These results demonstrate the existence of nanoscale phase separation as suggested by theoretical predictions based on nearly degenerate interface sub-bands associated with the Ti orbitals.12,13 The magnitude and temperature dependence of the paramagnetic response suggests that the vast majority of the electrons at the interface are localized, and do not contribute to transport measurements.3,6,7 In addition to the implications for magnetism, the existence of a 2D superconductor at an interface with highly broken inversion symmetry and a ferromagnetic landscape in the background suggests the potential for exotic superconducting phenomena.

Coexistence of magnetic order and two-dimensional superconductivity at LaAlO3/SrTiO3 interfaces

A two dimensional electronic system with novel electronic properties forms at the interface between the insulators LaAlO$_3$ and SrTiO$_3$. Samples fabricated until now have been found to be either magnetic or superconducting, depending on growth conditions. We combine transport measurements with high-resolution magnetic torque magnetometry and report here evidence of magnetic ordering of the two-dimensional electron liquid at the interface. The magnetic ordering exists from well below the superconducting transition to up to 200 K, and is characterized by an in-plane magnetic moment. Our results suggest that there is either phase separation or coexistence between magnetic and superconducting states. The coexistence scenario would point to an unconventional superconducting phase in the ground state.

Incommensurable state of a spin density wave and superconductivity in quasi-two-dimensional systems with an anisotropic energy spectrum

We investigate phase transitions in quasi-two-dimensional systems with an anisotropic energy spectrum and a deviation from the half-filling of the energy band (µ ≠ 0). We demonstrate the possibility of the transition of an insulator into a half-metallic state when the nesting condition is violated because the parameter µ ≠ 0 and of taking the umklapp processes into account. We obtain the basic equations for the parameters of the superconducting (Δ) and magnetic (M) orders and determine the conditions for the emergence of superconductivity on the background of a spin-density-wave state and also for the coexistence of superconductivity and magnetism. We show that the transition of a magnetic system into a superconducting state as the parameter µ increases can be a first-order phase transition at low temperatures. We also obtain an expression for the heat capacity jump C S -C N at T = T c , which depends on M and µ and differs essentially from the case of the Bardeen-Cooper-Schrieffer theory. We also consider the transformations related to the density of electron states of the relevant anisotropic system, which can undergo essential changes under pressure or doping.

Antiferromagnetic Order in Pauli-Limited Unconventional Superconductors

We develop a theory of the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in CeCoIn(5). We show that in Pauli-limited nodal superconductors the nesting of the quasiparticle pockets induced by Zeeman pair breaking leads to incommensurate AFM with the magnetic moment normal to the field. We compute the phase diagram and find a first order transition to the normal state at low temperatures, the absence of normal state AFM, and the coexistence of SC and AFM at high fields, in agreement with experiments. We also predict the existence of a new double-Q magnetic phase.

Coexistence of magnetic fluctuations and superconductivity in SmFe0.95Co0.05AsO seen in57Fe Mössbauer spectroscopy

The M\"ossbauer spectra (MS) of powder samples of SmFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO ($x$ $=$ 0.0, 0.05, and 0.1) were measured in applied fields up to 9 T and at temperatures up to 298 K. SmFeAsO is magnetically ordered with $T$${}_{N}$ $=$ 137 K and has a hyperfine magnetic field of (4.98 $\ifmmode\pm\else\textpm\fi{}$ 0.18) T at 4.2 K. In applied magnetic fields, the MS is consistent with a distribution of hyperfine magnetic fields of width ${H}_{\mathrm{applied}}+{H}_{\mathrm{hyperfine}}$. This arises because the angles between the direction of the ordered field in the crystallites making up the sample are randomly distributed about the direction of the applied field. The MS of the superconductors SmFe${}_{0.95}$Co${}_{0.05}$AsO ($T$${}_{C}\ensuremath{\simeq}$5 K) and SmFe${}_{0.9}$Co${}_{0.1}$AsO ($T$${}_{C}\ensuremath{\simeq}$17 K) are well described by a single peak from room temperature to 4.2 K indicating the absence of static magnetic order. However, the half width at half maximum, $\ensuremath{\Gamma}$, of the peak in SmFe${}_{0.95}$Co${}_{0.05}$AsO increases with decreasing temperature from its high temperature value, 0.13 mm/s at 25 K, to 0.25 mm/s at 10 K. No such temperature dependence is seen in SmFe${}_{0.9}$Co${}_{0.1}$AsO. We analyze this temperature dependence in terms of a fluctuating hyperfine magnetic field model whose frequency at 4.2 K is found to be $\ensuremath{\sim}$5--10 MHz, giving direct evidence of coexisting magnetic fluctuations and superconductivity at the interface in the phase diagram between the regions with magnetic and superconducting order. In a 5 T applied field, SmFe${}_{0.95}$Co${}_{0.05}$AsO is no longer superconducting; however, the temperature-dependent fluctuating magnetic field is still present and largely unchanged. The absence of fluctuations in superconducting SmFe${}_{0.9}$Co${}_{0.1}$AsO and their presence in superconducting SmFe${}_{0.95}$Co${}_{0.05}$AsO in zero applied field and in nonsuperconducting SmFe${}_{0.95}$Co${}_{0.05}$AsO at 5 T suggests that magnetic order is in competition with superconductivity in SmFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO.

Coexistence of Superconductivity and Ferromagnetism in Two Dimensions

Ferromagnetism is usually considered to be incompatible with conventional superconductivity, as it destroys the singlet correlations responsible for the pairing interaction. Superconductivity and ferromagnetism are known to coexist in only a few bulk rare-earth materials. Here we report evidence for their coexistence in a two-dimensional system: the interface between two bulk insulators, LaAlO(3) (LAO) and SrTiO(3) (STO), a system that has been studied intensively recently. Magnetoresistance, Hall, and electric-field dependence measurements suggest that there are two distinct bands of charge carriers that contribute to the interface conductivity. The sensitivity of properties of the interface to an electric field makes this a fascinating system for the study of the interplay between superconductivity and magnetism.

Coexistence of antiferromagnetic ordering and superconductivity in the Ba(Fe0.961Rh0.039)2As2compound studied by Mössbauer spectroscopy

The results of a $^{57}\mathrm{Fe}$ M\ossbauer spectroscopy study between 2.0 and 294 K of superconducting Ba(Fe${}_{0.961}$Rh${}_{0.039}$)${}_{2}$As${}_{2}$ are reported. The main component of the electric field gradient tensor at 294 K is shown to be positive and its increase with decreasing temperature is well described by a ${T}^{3/2}$ power-law relation. The shape of the M\ossbauer spectra below the N\'eel temperature ${T}_{N}=55.5(1)$ K is shown to result from the presence of doping-induced disorder rather than of incommensurate spin-density-wave order. The measured hyperfine magnetic field reaches its maximum value at the critical temperature ${T}_{c}=14$ K and then decreases by 4.2% upon further cooling to 2.0 K. This constitutes direct evidence of the coexistence of and competition between superconductivity and magnetic order. The extrapolated value of the Fe magnetic moment at 0 K is determined to be 0.35(1) ${\ensuremath{\mu}}_{B}$. The Debye temperature of Ba(Fe${}_{0.961}$Rh${}_{0.039}$)${}_{2}$As${}_{2}$ is found to be 357(3) K.

ON DISCOVERY OF THE FERROMAGNETIC SUPERCONDUCTOR UCoGe

We report the coexistence of ferromagnetic order and superconductivity in UCoGe at ambient pressure. The data obtained from the basic thermal, magnetic and transport properties on the macro and microscopic scale show that UCoGe is a weak ferromagnet with a Curie temperature TC = 3 K, and also, is a superconductor with a resistive transition temperature Ts = 0.8 K. Those present evidence that UCoGe is an unconventional superconductor and argue that superconductivity is mediated by critical ferromagnetic spin fluctuations.

Direct Observation of Local Potassium Variation and Its Correlation to Electronic Inhomogeneity in(Ba1−xKx)Fe2As2Pnictide

Local fluctuations in the distribution of dopant atoms are thought to cause the nanoscale electronic disorder or phase separation in pnictide superconductors. Atom probe tomography has enabled the first direct observations of dopant species clustering in a K-doped 122-phase pnictide. First-principles calculations suggest the coexistence of static magnetism and superconductivity on a lattice parameter length scale over a wide range of dopant concentrations. Our results provide evidence for a mixed scenario of phase coexistence and phase separation, depending on local dopant atom distributions.