Coexistence Of Superconductivity Research Articles

Effect of external magnetic field on the occupation probabilities of magneticsuperconductors

We present a theoretical model study for the coexistence of superconductivity (SC) and spin density wave (SDW) for high-Tc cuprates in the underdoped region before the onset of the superconductivity in the system. The analytic expressions for the temperature dependence of the SC and SDW order parameters are derived and solved self-consistently. It is observed that in the interplay region both the gap parameters exhibit very strong dependence of their gap values. The effect of the external magnetic field on the occupation probabilities for both itinerant and delocalized electrons is studied.

Coexistence of Superconductivity and Antiferromagnetism and its Effects on Crossover as to Correlation Strength in Hubbard Model

Abstract Using a variational Monte Carlo method, the interplay between antiferromagnetism and superconductivity is studied for the two- dimensional Hubbard model with a diagonal transfer t’. It is found with a refined function that the optimized state is sensitive to the values of model parameters, U/t, t’/t, and δ (doping rate), and becomes a coexisting state or a pure superconducting (SC) state. SC properties in the coexisting state also undergo considerable changes around the crossover point Uco(~10t) from a fragile BCS type for U Uco, in which pairs are formed through local exchange or doublon-holon processes, but supercurrent flows separately with the doped holes.

Hydrostatic-pressure tuning of magnetic, nonmagnetic, and superconducting states in annealed Ca(Fe1−xCox)2As2

We report on measurements of the magnetic susceptibility and electrical resistance under He-gas pressure on single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. We find that for properly heat-treated crystals with modest Co-concentration, $x$ = 0.028, the salient ground states associated with iron-arsenide superconductors, i.e., orthorhombic/antiferromagnetic (o/afm), superconducting, and nonmagnetic collapsed-tetragonal (cT) states can be accessed all in one sample with reasonably small and truly hydrostatic pressure. This is possible owing to the extreme sensitivity of the o/afm (for $T \leq$ $T_{s,N}$) and superconducting ($T \leq T_c$) states against variation of pressure, disclosing pressure coefficients of d$T_{s,N}$/d$P$ = -(1100 $\pm$ 50) K/GPa and d$T_{c}$/d$P$ = -(60 $\pm$ 3) K/GPa, respectively. Systematic investigations of the various phase transitions and ground states via pressure tuning revealed no coexistence of bulk superconductivity (sc) with the o/afm state which we link to the strongly first-order character of the corresponding structural/magnetic transition in this compound. Our results, together with literature results, indicate that preserving fluctuations associated with the o/afm transition to low enough temperatures is vital for sc to form.

Superconductivity appears in the vicinity of semiconducting-like behavior in CeO1−xFxBiS2

Resistive and magnetic properties have been measured in BiS${}_{2}$-based samples CeO${}_{1\ensuremath{-}x}$F${}_{x}$BiS${}_{2}$ with a systematic substitution of O with F ($0<x<0.6$). In contrast to the band-structure calculations, it is found that the parent phase of CeOBiS${}_{2}$ is a bad metal instead of a band insulator. By doping electrons into the system, it is surprising to find that superconductivity appears together with a semiconducting normal state. This evolution is clearly different from the cuprate and the iron pnictide systems, and is interpreted as approaching the Pomeranchuk transition with a von Hove singularity and the possible charge-density-wave instability. Furthermore, ferromagnetism, which may arise from the Ce magnetic moments, has been observed in the low-temperature region in all samples, suggesting the coexistence of superconductivity and ferromagnetism in the superconducting samples.

Influence of Connectivity on Vortex-Dynamic Properties of Polycrystalline NdFeAsO0.88F0.12 Superconductors

Polycrystalline NdFeAsO0.88F0.12 samples were prepared by both high pressure (HP) and ambient pressure (AP) methods. Magnetic hysteresis loops (MHLs) as well as magnetic relaxation were measured to investigate the vortex dynamic properties of the two samples. Magnetic relaxation rate S combined with effective pinning barrier energies U eff were calculated as a function of temperature and magnetic field. The results suggest that: (1) The samples with different connectivity display different properties of MHLs, indicating the coexistence of bulk superconductivity and granularity; (2) The different S(T) behaviors between sample AP and sample HP result from the transition from globality to granularity; (3) Field dependent S shows that the pinning mechanism of NdFeAsO0.88F0.12 can not be explained by collective pinning theory; (4) The anomalous temperature and magnetic field dependence of effective barrier energies as well as magnetic relaxation rate may be evoked by the competition between Bean-Livingstone (BL) surface pinning and bulk pinning.

NMR Study of the FFLO State and Magnetism in CeCoIn5

We present 115In NMR results in a novel high magnetic field and low temperature phase of CeCoIn5 where exotic superconductivity coexists with the incommensurate spin-density wave order. The quantitative analysis for the field evolution of newly-emerged low-energy quasiparticle density of states is consistent with the nodal plane formation which is expected in the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. The NMR spectra also suggest that the spatially uniform spin-density wave is induced in the FFLO phase.

Coexistence of superconductivity and ferromagnetism in iron pnictides

Superconductivity (SC) and long-range ferromagnetism (FM) are mutually antagonistic, thus in general SC does not coexist with FM. In this mini-review, however, we will show that such a coexistence can inhabit several iron pnictide systems, including EuFe2(As1−xPx)2, Eu(Fe1−xCox)2As2, Eu(Fe1−xRux)2As2, CeFe(As1−xPx)O0.95F0.05 and Sr2VFeAsO3. We will briefly discuss the possible reasons and consequences of the coexistence of 3d-electron SC and 4f-electron FM.

Possible multigap superconductivity and magnetism in single crystals of superconducting La2Pt3Ge5and Pr2Pt3Ge5

We herein describe our investigation of the superconducting and magnetic properties of the rare-earth ternary germanide intermetallic compounds La${}_{2}$Pt${}_{3}$Ge${}_{5}$ and Pr${}_{2}$Pt${}_{3}$Ge${}_{5}$. Single crystals of La${}_{2}$Pt${}_{3}$Ge${}_{5}$ and Pr${}_{2}$Pt${}_{3}$Ge${}_{5}$ were synthesized using the high-temperature metal flux method. Both types of crystal formed in a U${}_{2}$Co${}_{3}$Si${}_{5}$-type orthorhombic structure (space group $Ibam$). La${}_{2}$Pt${}_{3}$Ge${}_{5}$ showed the onset of superconducting phase transition at ${T}_{c}$ $=$ 8.1 K, which, to the best of our knowledge, is the highest ${T}_{c}$ of all the $R$${}_{2}$$M$${}_{3}$$X$${}_{5}$ ($R$ $=$ rare-earth elements, $M$ $=$ transition metal, and $X$ $=$ $s\text{\ensuremath{-}}p$ metal) superconductors, and from the specific heat data, it was found to have multigap superconductivity. Pr${}_{2}$Pt${}_{3}$Ge${}_{5}$ showed both a superconducting phase transition at ${T}_{c}$ $=$ 7.8 K and two antiferromagnetic transitions at ${T}_{N1}$ $=$ 3.5 K and ${T}_{N2}$ $=$ 4.2 K, which indicates the coexistence of superconductivity and magnetism. However, the correlation between the superconductivity and the magnetism was too weak to be observed. In its normal state, Pr${}_{2}$Pt${}_{3}$Ge${}_{5}$ revealed strong magnetic anisotropy, probably due to the crystalline electric field effect.

Nonmagnetic Γ3 doublet ground state in a caged compound PrRh2Zn20

We have prepared single crystals of PrRh2Zn20, which is isostructural and isoelectronic to PrIr2Zn20 where coexistence of superconductivity and antiferroquadrupolar order has been recently found. The electrical resistivity ρ, magnetic susceptibility χ, isothermal magnetization M, and specific heat C have been measured. Hysteretic behavior appears in ρ(T) for 170 K < T < 470 K, suggesting a structural phase transition. On cooling below 10 K, χ approaches a constant value, indicating a non-magnetic ground state. Anisotropic behavior in M(B) at 1.8 K for B > 2 T and a broad peak in C(T) at around 10 K are reproduced by the two crystal-field levels split by 32 K; a nonmagnetic Γ3 doublet ground state and a magnetic Γ4 triplet excited state.

Microscopic Coexistence of Superconductivity and Antiferromagnetism in UnderdopedBa(Fe1−xRux)2As2

We use $^{75}$As nuclear magnetic resonance (NMR) to investigate the local electronic properties of Ba(Fe$_{1-x}$Ru$_{x}$)$_2$As$_2$ ($x =$ 0.23). We find two phase transitions, to antiferromagnetism at $T_N \approx$ 60 K and to superconductivity at $T_C \approx$ 15 K. Below $T_N$, our data show that the system is fully magnetic, with a commensurate antiferromagnetic structure and a moment of 0.4 $\mu_B$/Fe. The spin-lattice relaxation rate $1/^{75}T_1$ is large in the magnetic state, indicating a high density of itinerant electrons induced by Ru doping. On cooling below $T_C$, $1/^{75}T_1$ on the magnetic sites falls sharply, providing unambiguous evidence for the microscopic coexistence of antiferromagnetism and superconductivity.

The coexistence of superconductivity and ferromagnetism in nano-scale metallic grains

A nano-scale metallic grain in which the single-particle dynamics are chaotic is described by the so-called universal Hamiltonian. This Hamiltonian includes a superconducting pairing term and a ferromagnetic exchange term that compete with each other: pairing correlations favor minimal ground-state spin, while the exchange interaction favors maximal spin polarization. Of particular interest is the fluctuation-dominated regime where the bulk pairing gap is comparable with or smaller than the single-particle mean level spacing and the Bardeen–Cooper–Schrieffer theory of superconductivity breaks down. Superconductivity and ferromagnetism can coexist in this regime. We identify signatures of the competition between superconductivity and ferromagnetism in a number of quantities: ground-state spin, conductance fluctuations when the grain is weakly coupled to external leads and the thermodynamic properties of the grain, such as heat capacity and spin susceptibility.

Anomalous local magnetic field distribution and strong pinning in CaFe1.94Co0.06As2 single crystals

Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows anomalous remnant magnetization within Meissner-like regions of the superconductor. The unconventional shape of the local magnetization hysteresis loop suggests admixture of superconducting and magnetic fractions governing the response. Near the superconducting transition temperature the local magnetic field exceeds the applied field resulting in a diamagnetic to positive magnetization transformation. The observed anomalies in the local magnetic field distribution are accompanied with enhanced bulk pinning in the CaFe1.94Co0.06As2 single crystals. We propose that our results suggest a coexistence of superconductivity and magnetic correlations.

Termination control of electronic phases in oxide thin films and interfaces: LaAlO 3 /SrTiO 3 (001)

A wealth of intriguing properties emerge in the seemingly simple system composed of the band insulators LaAlO 3 and SrTiO 3 such as a two-dimensional electron gas, superconductivity and magnetism. In this paper, we review the current insight obtained from first principles calculations on the mechanisms governing the behaviour of thin LaAlO 3 films on SrTiO 3 (001). In particular, we explore the strong dependence of the electronic properties on the surface and interface termination, the finite film thickness, lattice polarization and defects. A further aspect that is addressed is how the electronic behaviour and functionality can be tuned by an SrTiO 3 capping layer, adsorbates and metallic contacts. Lastly, we discuss recent reports on the coexistence of magnetism and superconductivity in this system for what they might imply about the electronic structure of this system.

Real-Space Coexistence of the Melted Mott State and Superconductivity in Fe-Substituted1T−TaS2

We have performed high-resolution angle-resolved photoemission spectroscopy of layered chalcogenide 1T-Fe(x)Ta(1-x)S(2) which undergoes a superconducting transition in the nearly commensurate charge-density-wave phase (melted Mott phase). We found a single electron pocket at the Brillouin-zone center in the melted Mott phase, which is created by the backfolding of bands due to the superlattice potential of charge-density-wave. This electron pocket appears in the x region where the samples show superconductivity, and is destroyed by the Mott- and Anderson-gap opening. Present results suggest that the melted Mott state and the superconductivity coexist in real space, providing a new insight into the interplay between electron correlation, charge order, and superconductivity.

Thermodynamic phase diagram, phase competition, and uniaxial pressure effects in BaFe2(As1−xPx)2studied by thermal expansion

High-resolution thermal-expansion and specific-heat data of isovalently substituted single-crystalline BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ ($0\ensuremath{\le}x\ensuremath{\le}0.33$, $x=1$) are presented. We show that crystals can be detwinned in situ in the capacitance dilatometer, allowing a study of all three independent crystallographic directions. From the thermal-expansion data, we determine the phase diagram via a thermodynamic probe, study the coupling of the spin-density wave (SDW) and superconducting order parameters, and determine various pressure dependencies of the normal and superconducting states. Our results show that in the underdoped region, superconductivity and SDW order coexist and compete with each other. The resulting phase diagram, however, exhibits a smaller coexistence region of SDW and superconductivity with a steeper rise of ${T}_{c}$ on the underdoped side than in, e.g., Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$. On the overdoped side, where there is no sign of SDW order, the lattice parameters respond to superconductivity in much the same way as to the SDW on the underdoped side, which demonstrates the intimate connection between both kinds of order. Using thermodynamic relations, the uniaxial pressure derivatives of the superconducting critical temperature and the electronic Sommerfeld coefficient are determined from our thermal-expansion data together with the specific-heat data. We find that uniaxial pressure is proportional to P substitution and that the electronic density of states has a maximum at optimal doping. Overall, the coupling of the SDW and superconducting order to the lattice parameters of BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ is found to be qualitatively very similar to that of the well-studied, supposedly electron-doped Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ system.

Iron-based superconductors: Magnetism, superconductivity, and electronic structure (Review Article)

Angle resolved photoemission spectroscopy (ARPES) reveals the features of the electronic structure of quasi-two-dimensional crystals which are crucial for spin and charge ordering and determine the mechanisms of electron–electron interactions, including superconducting pairing. The newly discovered iron-based superconductors (FeSC) promise interesting physics stemming, on one hand, from a coexistence of superconductivity and magnetism and, on the other, from a complex multi-band electronic structure. In this review I want to offer a simple introduction to the physics of FeSC, and to argue that all the complexity of FeSC properties is encapsulated in their electronic structure. For many compounds, this structure has been determined on the basis of numerous ARPES experiments and agrees reasonably well with the results of band structure calculations. Nevertheless, the existing small differences may help to understand the mechanisms of magnetic ordering and superconducting pairing in FeSC.

Superconducting and Magnetic Properties of Polycrystalline [(Bi,Pb)2Sr2Ca2Cu3O x ]1−x (La0.7Sr0.3MnO3) x Thick Films Grown Using MQA Method

Polycrystalline thick films with the nominal composition [(Bi,Pb)2Sr2Ca2Cu3Ox]1−x(La0.7Sr0.3MnO3)x (x=0, 0.01, 0.03, 0.05) were prepared by the melting-quenching-annealing (MQA) method on (001)-oriented LaAlO3 (LAO) substrates. The XRD patterns show that the samples are composites consisting of (Bi,Pb)2Sr2Ca2Cu3Ox and La0.7Sr0.3MnO3 particles with average grain sizes of ∼60 and ∼30 nm, respectively. From electric transport measurements, the superconducting onset temperature, Tonset, and superconducting critical temperature, Tc, ended up being ∼80 and ∼60 K, respectively. The depression of Tc may be attributed to the proximity effect between ferromagnetism and superconductivity. The M–H hysteresis loops indicate the coexistence of ferromagnetism and superconductivity in the films. The magnetic field dependence of the superconducting current density and the volume pinning force Fp=Jc×B were obtained by applying Bean’s model to the isothermal M–H loops.

Coexistence of spin-triplet superconductivity with magnetic ordering in an orbitally degenerate system: Hartree-Fock-BCS approximation revisited

The Hund's-rule-exchange induced and coexisting spin-triplet paired and magnetic states are considered within the doubly degenerate Hubbard model with interband hybridization. The Hartree-Fock approximation combined with the Bardeen-Cooper-Schrieffer (BCS) approach is analyzed for the case of square lattice. The calculated phase diagram contains regions of stability of the spin-triplet superconducting phase coexisting with either ferromagnetism or antiferromagnetism, as well as a pure superconducting phase. The influence of the inter-site hybridization on the stability of the considered phases, as well as the temperature dependence of both the magnetic moment and the superconducting gaps, are also discussed. Our approach supplements the well known phase diagrams containing only magnetic phases with the paired triplet states treated on the same footing. We also discuss briefly how to include the spin fluctuations within this model with real space pairing.

Semimetallic Paramagnetic Nano‐Bi2Ir and Superconducting Ferromagnetic Nano‐Bi3Ni by Microwave‐Assisted Synthesis and Room Temperature Pseudomorphosis

Abstract Uniform nanocrystals of the intermetallic compounds Bi2Ir (diameter ≥ 50 nm) and Bi3Ni (typical size 200 × 600 nm) were obtained by a microwave‐assisted polyol process at 240 °C. The method was also applied to the spatially confined reaction environment in the microporous exo‐template SBA‐15 resulting in Bi3Ni particles of about 6 nm. Non‐crystalline bundles of parallel Bi3Ni nanofibres that have an individual diameter of less than 1 nm were obtained by reductive pseudomorphosis of the subiodide Bi12Ni4I3 at room temperature. Magnetic susceptibility measurements demonstrate coexistence of ferromagnetism and superconductivity in a single phase for the nanostructured Bi3Ni materials. Curie temperature, coercive field, remnant magnetization, saturation moment, diamagnetic screening, and critical field vary with particle size. The crystal structure of Bi2Ir was determined by Rietveld refinement of powder X‐ray diffraction data. Bi2Ir crystallizes in the monoclinic arsenopyrite type (space group P21/c), a superstructure of the markasite type, with a = 690.11(1), b = 678.85(1), c = 696.17(1) pm, and β = 116.454(1)°. In contrast to most of the other phases of this type, the Bi2Ir is not a diamagnetic semiconductor but a weakly paramagnetic semimetal. Conductivity measurements down to 4 K and magnetization measurements in a field of μ0H = 10 mT down to 1.8 K give no evidence for a transition into the superconducting state. Bonding analysis shows prevailing contribution of Bi–Bi interactions to the conduction, whereas Bi–Ir bonding is mostly covalent and localized.

The electronic phase diagrams of the Eu(Fe0.81Co0.19)2As2 superconductor

The magnetic and superconducting properties of an Eu(Fe0.81Co0.19)2As2 single crystal are investigated by means of ac magnetic susceptibility, dc magnetization, specific heat, transverse resistivity and Hall effect measurements in magnetic fields up to 9 T, applied parallel and perpendicular to the c-axis. The compound exhibits the coexistence of magnetism and superconductivity (SC), characterized by structural distortion (SD) and/or spin-density-wave (SDW) ordering at TSD/SDW = 78 ± 4 K, canted-antiferromagnetic (C-AF) ordering at the Néel temperature TN = 16.5 ± 0.5 K and SC at the critical temperature Tc = 5.3 ± 0.2 K at zero field. Upon applying fields both the C-AF and SC states evolve in an unconventional manner. Magnetic field distinctly affects the spin canting, resulting in separation of the C-AF into two new phases: the C-AF and ferromagnetic (F) ones. The unusual behavior of the SC state produces field-induced SC in the H⊥c configuration as an outcome of the weakening orbital pair-breaking effect. From the experimental data we derive the field-temperature phase diagrams for Eu(Fe0.81Co0.19)2As2. A comparison of experimental results is made with theory developed for type II superconductors and then some important thermodynamic parameters characteristic of the superconducting state of Eu(Fe0.81Co0.19)2As2 are deduced such as the specific heat jump at Tc, ΔCp(Tc)/γnTc, the electron–phonon coupling constant λe–ph, the upper critical field Hc2, coherence length ξ, the Fermi wave-vector kF, effective mass m*, Hall mobility μH, magnetic penetration depth λ and the Ginzburg–Landau parameter κ.