W. Yu, J. S. Higgins, P. Bach, R. L. Greene
We present magnetotransport evidence for antiferromagnetism in films of the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_4$. Our results show clear signature of static antiferromagnetism up to optimal doping x=0.15, with a quantum phase transition close to x=0.16, and a coexistence of static antiferromagnetism and superconductivity for 0.12$\le$x$\le$0.15.
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The authors discuss the possibility of coexistence of antiferromagnetism and triplet superconductivity as a particular example of a broad class of systems where the interplay of magnetism and superconductivity is important. This paper focuses on the case of quasi-one-dimensional metals, where it is known experimentally that antiferromagnetism is in close proximity to triplet superconductivity in the temperature versus pressure phase diagram. Over a narrow range of pressures, the authors propose an intermediate non-uniform phase consisting of alternating insulating antiferromagnetic and triplet…
On the basis of the periodic Anderson model the microscopic Ginzburg-Landau equations for heavy-fermion superconductors in the coexistence phase of superconductivity and antiferromagnetism have been derived. The obtained expressions are valid in the vicinity of quantum critical point of heavy-fermion superconductors when the onset temperatures of antiferromagnetism and superconductivity are sufficiently close to each other. It is shown that the formation of antiferromagnetic ordering causes a decrease of the critical temperature of superconducting transition and order parameter in the phase of…
We discuss the coexistence of antiferromagnetism and d-wave superconductivity within the so-called statistically-consistent Gutzwiller approximation (SGA) applied to the t-J-U model. In this approach, the averages calculated in a self-consistent manner coincide with those determined variationally. Such consistency is not guaranteed within the standard renormalized mean field theory. With the help of SGA, we show that for the typical value J/|t| = 1/3, coexistence of antiferromagnetism (AF) and superconductivity (SC) appears only for U/|t| > 10.6 and in a very narrow range of doping (δ< 0.006) …
We report simultaneous NMR and electrical transport experiments in the pressure range near the boundary of the antiferromagnetic spin density wave (SDW) insulator and the metal-lic/superconducting (SC) phase in (TMTSF)2PF6. Measurements indicate a tricritical point separat-ing a line of second order SDW/metal transitions from a line of first order SDW/metal(SC) transi-tions with coexistence of macroscopic regions of SDW and metal(SC) order, with little mutual in-teraction but strong hysteretic effects. NMR results quantify the fraction of each phase.
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 $μ_B$/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 t…
The two-dimensional t-J model in a staggered field is studied by exact diagonalization of small clusters. For the low-hole-density region and a realistic value of J/t, it is found that the presence of a staggered field strengthens the attraction between two holes. With increasing field, the d_{x^2-y^2}-wave superconducting correlations are enhanced while the extended-s-wave ones hardly change. This implies that coexistence of the d_{x^2-y^2}-wave superconducting order and the commensurate antiferromagnetic order occurs in a staggered field.
Based on a hybrid interlayer coupling mechanism, we study the coexistence of superconductivity (SC) and antiferromagnetism (AFM) in trilayer cuprates. By introducing an interlayer magnetic scattering term, we solve the multilayer $t{-}J$ model with Josephson coupling under the framework of Gutzwiller projection. We show that both the SC and AFM orders in the multilayered system are enhanced and the range of AFM order is extended. The layer configuration of d-wave pairing gap and AFM order further plays an essential role in determining the interlayer magnetic and superconducting coupling phase …
We argue the emergence of nodal excitations due to the coupling with static antiferromagnetic order in fully-gapped superconducting states in both cases with and without inversion symmetry. This line node structure is not accompanied with the sign change of the superconducting gap, in contrast to usual unconventional Cooper pairs with higher angular momenta. In the case without inversion symmetry, the stability of the nodal excitations crucially depends on the direction of the antiferromagnetic staggered magnetic moment. A possible realization of this phenomenon in CePt$_3$Si is discussed.
We report on detailed ac calorimetry measurements under high pressure and magnetic field of CeRhIn5. Under hydrostatic pressure the antiferromagnetic order vanishes near p_c*=2 GPa due to a first order transition. Superconductivity is found for pressures above 1.5 GPa inside the magnetic ordered phase. However, the superconductivity differ from the pure homogeneous superconducting ground state above 2 GPa. The application of an external magnetic field H || ab induces a transition inside the superconducting state above pc* which is strongly related to the re-entrance of the antiferromagnetism w…
We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn$_5$ under pressure $P=1.75$ GPa. The onset of the magnetic order is evidenced from a clear split of $^{115}$In-NQR spectrum due to the spontaneous internal field below the Néel temperature $T_N=2.5$ K. Simultaneously, bulk SC below $T_c=2.0$ K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic…
We use c-axis resistivity and magnetoresistance measurements to study the interplay between antiferromagnetic (AF) and superconducting (SC) ordering in underdoped RBa_2Cu_3O_{6+x} (R = Lu, Y) single crystals. Both orders are found to emerge from an anisotropic 3D metallic state, upon which antiferromagnetism opposes superconductivity by driving the doped holes towards localization. Despite the competition, the superconductivity sets in before the AF order is completely destroyed and coexists with latter in a certain range of hole doping. We find also that strong magnetic fields affect the AF-S…
This paper was withdrawn by the authors due to the belated discovery of a published paper showing similar results.
The possibility of coexistence of superconductivity (SC) and antiferromagnetic long range order (AFLRO) of the two-dimensional extended $t-J$ model in the very underdoped region is studied by the variational Monte-Carlo (VMC) method. In addition to using previously studied wave functions, a recently proposed new wave function generated from the half-filled Mott insulator is used. For hole-doped systems, the phase boundary between AFLRO and $d-$wave SC for the physical parameters, $J/t=0.3$, $t'/t=-0.3$ and $t''/t=0.2$, is located near hole density $δ_c = 0.06$, and there is {\it no} coexistenc…
We propose a class of wave functions that provide a unified description of antiferromagnetism and d-wave superconductivity in (doped) Mott insulators. The wave function has a Jastrow form and prohibits double occupancies. In the absence of holes, the wave function describes antiferromagnetism accurately. Off diagonal long range order develops at finite doping and the superconducting order parameter has d-wave symmetry. We also show how nodal quasiparticles and neutral spin excitations can be constructed from this wave function.
We report on the basis of Cu-NQR measurements that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu_{2}(Si_{1-x}Ge_{x})_{2}, once a tiny amount of 1%Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu2Si2 where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in either nearly homogeneous or slightly Ge substituted CeCu2Si2 are accountable based on the SO(5) theory that unifies the SC and AF. We suggest…
Materials with exceptional magnetism and superconductivity usually conceive emergent physical phenomena. Here, we investigate the physical properties of the (Eu,La)FeAs2 system with double magnetic sublattices. The parent EuFeAs2 shows anisotropy-associated magnetic behaviors, such as Eu-related moment canting and exchange bias. Through La doping, the magnetic anisotropy is enhanced with ferromagnetism of Eu2+ realized in the overdoped region, and a special exchange bias of the superposed ferromagnetic/superconducting loop revealed in Eu0.8La0.2FeAs2. Meanwhile, the Fe-related antiferromagneti…
Motivated by recent experimental study of antiferromagnetic property of honeycomb compound In$_{3}$Cu$_{2}$VO$_{9}$ [Yan \textit{et al.}, PRB \textbf{85}, 085102 (2012)], we explore possible superconductivity and its coexistence with antiferromagnetism. We use the t-t$^\prime$-J model on the honeycomb lattice as our starting point and employ the slave-boson mean-field theory. In the antiferromagnetic normal state, the characteristic doping evolution of Fermi surface shows that only one effective singe band is active, which suggests that the potential pairing symmetry is the time-reversal symme…
We discuss the $t$-$J$-$U$ model in the mean-field approximation. The role of spin-exchange coupling $J$ and the second nearest hopping $t'$ are examined in the context of the coexistence of superconductivity (SC) and antiferromagnetism (AF). Stability of the phases is studied with respect to temperature. The coexistence region exists for the sufficiently large Coulomb repulsion ($U>U_{cr}$), and in the vicinity of the half-filled band (hole doping $δ< δ_{cr}$). The critical hole doping is relatively small ($δ_{cr} \approx 0.006$ for $J/|t| = 1/3$) and linear with respect to $J$. The decrease …
We report systematic measurements of ac-susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time ($T_1$) on the pressure ($P$)- induced heavy-fermion (HF) superconductor CeRhIn$_5$. The temperature ($T$) dependence of $1/T_1$ at $P$ = 1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at $T_N = 2.8$ K and $T^{MF}_c$ = 0.9 K. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below $T_c^{onset} = 2$ K, but $T_c^{MF} = 0.9$ K, fo…
On the basis of the Hubbard model, we extend the fluctuation-exchange (FLEX) approach to investigating the properties of antiferromagnetic (AF) phase in electron-doped cuprate superconductors. Furthermore, by incorporating the effect of scatterings due to the disordered dopant-atoms into the FLEX formalism, our numerical results show that the antiferromagnetic transition temperature, the onset temperature of pseudogap due to spin fluctuations, the spectral density of the single particle near the Fermi surface, and the staggered magnetization in the AF phase as a function of electron doping can…
A self-doped bilayer t-t'-J model of an electron- and a hole-doped planes is studied by the slave-boson mean-field theory. A hopping integral between the differently doped planes, which are generated by a site potential, are renormalized by the electron-electron correlation. We find coexistent phases of antiferromagnetic (AFM) and superconducting orders, although the magnitudes of order parameters become more dissimilar in the bilayer away from half-filling. Fermi surfaces (FS's) with the AFM order show two pockets around the nodal and the anti-nodal regions. These results look like a composit…
We theoretically investigate the coexistence of antiferromagnetism and superconductivity in the iron-based superconductors by using the mean-field theory for two- and three-orbital models. We find that both the s_{+-}-wave and s_{++}-wave superconductivity can coexist with antiferromagnetism in the two models. On Dirac Fermi surfaces emerging in the antiferromagnetic phase, a superconducting-gap function has a node for s_{++} wave but is nodeless for s_{+-} wave. On the other hand, the gap function on non-Dirac Fermi surfaces is either nodeless or accidentally nodal, depending on the parameter…
We report Cu- and F-NMR studies on a four-layered high-temperature superconductor Ba2Ca3Cu4O8F2(0234F(2.0)) with apical fluorine (F-1), an undoped 55 K-superconductor with a nominal Cu2+ valence on average. We reveal that this compound exhibits the antiferromagnetism (AFM) with a Neel temperature TN=100 K despite being a Tc= 55 K-superconductor. Through a comparison with a related tri-layered cuprate Ba2Ca2Cu3O6F2 (0223F(2.0)), it is demonstrated that electrons are transferred from the inner plane (IP) to the outer plane (OP) in 0234F(2.0) and 0223F(2.0), confirming the self-doped high-tempera…
Magnetization, nuclear magnetic resonance, high-resolution x-ray diffraction and magnetic field-dependent neutron diffraction measurements reveal a novel magnetic ground state of Ba{0.60}K{0.40}Mn2As2 in which itinerant ferromagnetism (FM) below a Curie temperature TC = 100 K arising from the doped conduction holes coexists with collinear antiferromagnetism (AFM) of the Mn local moments that order below a Neel temperature TN = 480 K. The FM ordered moments are aligned in the tetragonal ab-plane and are orthogonal to the AFM-ordered Mn moments that are aligned along the c-axis. The magnitude an…
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 l…
In the generic phase diagram of heavy fermion systems, tuning an external parameter such as hydrostatic or chemical pressure modifies the superconducting transition temperature. The superconducting phase forms a dome in the temperature-tuning parameter phase diagram, which is associated with a maximum of the superconducting pairing interaction. Proximity to antiferromagnetism suggests a relation between the disappearance of antiferromagnetic order and superconductivity. We combine muon spin rotation, neutron scattering, and x-ray absorption spectroscopy techniques to gain access to the magneti…
We have performed a systematic angle-resolved photoemission study of as-grown and oxygen-reduced Pr$_{2-x}$Ce$_x$CuO$_4$ and Pr$_{1-x}$LaCe$_{x}$CuO$_4$ electron-doped cuprates. In contrast to the common belief, neither the band filling nor the band parameters are significantly affected by the oxygen reduction process. Instead, we show that the main electronic role of the reduction process is to remove an anisotropic leading edge gap around the Fermi surface. While the nodal leading edge gap is induced by long-range antiferomagnetic order, the origin of the antinodal one remains unclear.
A high-quality PrFeAsO$_{1-δ}$ single crystal ($T_{\rm c}=44$ K) has been investigated by the magnetic torque. Antiferromagnetism of the Pr$^{3+}$ ions was found to coexist with superconductivity in PrFeAsO$_{1-δ}$ at temperatures below $T_\mathrm{N}=14$ K. We predict a magnetic structure that is not in accordance with earlier neutron studies performed using polycrystalline nonsuperconducting specimens. As the temperature decreases, the superconducting anisotropy $γ\sim 4$ of PrFeAsO$_{1-δ}$ increases near $T_c$ and tends to decrease slightly at lower temperatures.
The Ce compounds CeCoIn$_5$ and CeRhIn$_5$ are ideal model systems to study the competition of antiferromagnetism (AF) and superconductivity (SC). Here we discuss the pressure--temperature and magnetic field phase diagrams of both compounds. In CeRhIn$_5$ the interesting observation is that in zero magnetic field a coexistence AF+SC phase exist inside the AF phase below the critical pressure $p_{\rm c}^\star \approx 2$ GPa. Above $p_{\rm c}^\star$ AF is suppressed in zero field but can be re-induced by applying a magnetic field. The collapse of AF under pressure coincides with the abrupt chang…
On the basis of the Hubbard model, we present the formulation of antiferromagnetism in electron-doped cuprates using the fluctuation-exchange approach. Taking into account the spin fluctuations in combination with the impurity scattering effect due to the randomly distributed dopant-atoms, we investigate the magnetic properties of the system. It is shown that the antiferromagnetic transition temperature, the onset temperature of the pseudogap formation, the single particle spectral density, and the staggered magnetization obtained by the present approach are in very good agreement with the exp…
By performing first-principles electronic structure calculations and analyzing effective magnetic model of alkali-doped iron selenides, we show that the materials without iron vacancies should approach a novel checkerboard phase in which each four Fe sites group together in tetragonal structure. The checkerboard phase is the ground state with a block antiferromagnetic (AFM) order and a small charge density wave order in the absence of superconductivity. Both of them can also coexist with superconductivity. The results explain mysterious $2\times 2$ ordered patterns and hidden orders observed i…
Whether or not a physical property can be enhanced in an inhomogeneous system compared with its homogeneous counterpart is an intriguing fundamental question. We provide a concrete example with positive answer by uncovering a remarkable enhancement of both antiferromagnetic (AF) structure factor and $d$-wave pairing tendency in the doped staggered periodic Anderson model (PAM) with two alternating inequivalent local moments. The common thread of these enhancement is found to originate from the generic "self-averaging" effect and non-monotonic dependence of the corresponding physical quantity i…
We discuss the possibility of coexistence of spin density wave (antiferromagnetism) and triplet superconductivity as a particular example of a broad class of systems where the interplay of magnetism and superconductivity is important. We focus on the case of quasi-one-dimensional metals, where it is kown that antiferromagnetism is in close proximity to triplet superconductivity in the pressure versus temperature phase diagram. Over a range of pressures, we propose an intermediate non-uniform phase consisting of antiferromagnetic and triplet superconducting orders. In the coexistence region, we…
We report the transport, thermodynamic, $μ$SR and neutron study of the Ca$_{0.74(1)}$La$_{0.26(1)}$(Fe$_{1-x}$Co$_{x}$)As$_{2}$ single crystals, mapping out the temperature-doping level phase diagram. Upon Co substitution on the Fe site, the structural/magnetic phase transitions in this 112 compound are suppressed and superconductivity up to 20 K occurs. Our measurements of the superconducting and magnetic volume fractions show that these two phases coexist microscopically in the underdoped region, in contrast to the related 10-3-8 Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_x$)$_{2}$As$_{2}$)$…
Emergency of superconductivity at the instabilities of antiferromagnetism (AFM), spin/charge density waves has been widely recognized in unconventional superconductors. In copper-oxide superconductors, spin fluctuations play a predominant role in electron pairing with electron dopants yet composite orders veil the nature of superconductivity for hole-doped family. However, in electron-doped ones the ending point of AFM is still in controversy for different probes or its sensitivity to oxygen content. Here, by carefully tuning the oxygen content, a systematic study of Hall signal and magnetores…
We performed systematic angle-resolved photoemission spectroscopy measurements $in$-$situ$ on $T'$-${\rm La}_{2-x}{\rm Ce}_x\rm {CuO}_{4\pmδ}$ (LCCO) thin films over the extended doping range prepared by the refined ozone/vacuum annealing method. Electron doping level ($n$), estimated from the measured Fermi surface volume, varies from 0.05 to 0.23, which covers the whole superconducting dome. We observed an absence of the insulating behavior around $n \sim$ 0.05 and the Fermi surface reconstruction shifted to $n \sim$ 0.11 in LCCO compared to that of other electron-doped cuprates at around 0.…
We report the novel pressure(P)-temperature(T) phase diagrams of antiferromagnetism (AF) and superconductivity (SC) in CeRhIn$_5$, CeIn$_3$ and CeCu$_2$Si$_2$ revealed by the NQR measurement. In the itinerant helical magnet CeRhIn$_5$, we found that the Néel temperature $T_N$ is reduced at $P \geq$ 1.23 GPa with an emergent pseudogap behavior. The coexistence of AF and SC is found in a narrow P range of 1.63 - 1.75 GPa, followed by the onset of SC with line-node gap over a wide P window 2.1 - 5 GPa. In CeIn$_3$, the localized magnetic character is robust against the application of pressure up …
We discuss the possibility of coexistence of spin density wave (antiferromagnetism) and triplet superconductivity as a particular example of a broad class of systems where the interplay of magnetism and superconductivity is important. We focus on the case of quasi-one-dimensional metals, where it is known experimentally that antiferromagnetism is in close proximity to triplet superconductivity in the temperature versus pressure phase diagram. Over a narrow range of pressures, we propose an intermediate non-uniform phase consisting of alternating antiferromagnetic and triplet superconducting st…
Using mean-field theory, we illustrate the long-range Coulomb effect on the antiferromagnetism in the electron-doped cuprates. Because of the Coulomb exchange effect, the magnitude of the effective next nearest neighbor hopping parameter increases appreciably with increasing the electron doping concentration, raising the frustration to the antiferromagnetic ordering. The Fermi surface evolution in the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$ and the doping dependence of the onset temperature of the antiferromagnetic pseudogap can be reasonably explained by the present consideration.
Strongly correlated systems exhibit a rich phenomenology due to the antagonism of a diversity of ordered phases. The aftermath of this interplay can lead to a coexistence which takes place at a microscopic level, or, a phase separation in which non-overlapping single-order domains extend throughout the material. In most cases it appears experimentally challenging to disentangle the two scenarios, unless, there exist robust and measurable properties particular to only one of the two types of coexistence. This is for instance the case when the type of coexistence decides on the appearance of top…
We study the interplay between antiferromagnetism and superconductivity in a generalized infinite-$U$ Anderson lattice, where both superconductivity and antiferromagnetic order are introduced phenomenologically in mean field theory. In a certain regime, a quantum phase transition is found which is characterized by an abrupt expulsion of magnetic order by d-wave superconductivity, as externally applied pressure increases. This transition takes place when the d-wave superconducting critical temperature, $T_c$, intercepts the magnetic critical temperature, $T_m$, under increasing pressure. Calcul…
The heavy fermion Ce(Rh,Ir)In5 system exhibits properties that range from an incommensurate antiferromagnet on the Rh-rich end to an exotic superconductor on the Ir-rich end of the phase diagram. At intermediate composition where antiferromagnetism coexists with superconductivity, two types of magnetic order are observed: the incommensurate one of CeRhIn5 and a new, commensurate antiferromagnetism that orders separately. The coexistence of f-electron superconductivity with two distinct f-electron magnetic orders is unique among unconventional superconductors, adding a new variety to the usual …
Local antiferromagnetism coexists with superconductivity in the cuprates. Charge segregation provides a way to reconcile these properties. Direct evidence for modulated spin and charge densities has been found in neutron and X-ray scattering studies of Nd-doped La(2-x)Sr(x)CuO(4). Here we discuss the nature of the modulation, and present some new results for a Zn-doped sample. Some of the open questions concerning the connections between segregation and superconductivity are described.
The asymmetry between electron and hole doping remains one of the central issues in high-temperature cuprate superconductivity, but our understanding of the electron-doped cuprates has been hampered by apparent discrepancies between the only two known families: Re2-xCexCuO4 and A1-xLaxCuO2. Here we report in situ angle-resolved photoemission spectroscopy measurements of epitaxially-stabilized films of Sr1-xLaxCuO2 synthesized by oxide molecular-beam epitaxy. Our results reveal a strong coupling between electrons and (pi,pi) antiferromagnetism that induces a Fermi surface reconstruction which p…
By performing high-pressure single-crystal neutron diffraction measurements, the evolution of structure and magnetic ordering in EuFe2As2 under hydrostatic pressure were investigated. Both the tetragonal-toorthorhombic structural transition and the Fe spin-density-wave (SDW) transition are gradually suppressed and become decoupled with increasing pressure. The antiferromagnetic order of the Eu sublattice is, however, robust against the applied pressure up to 24.7 kbar, without showing any change of the ordering temperature. Under the pressure of 24.7 kbar, the lattice parameters of EuFe2As2 di…
We have performed an angle-resolved photoemission study of the electron-doped high-Tc superconductors (HTSCs) Sm_{1.85}Ce_{0.15}CuO_{4} (SCCO) and Eu_{1.85}Ce_{0.15}CuO_{4} (ECCO). Around the nodal point (pi/2, pi/2), we observed an energy gap at the Fermi level (EF) for both samples, resulting from the strong effects of antiferromagnetism. The magnitude of this gap in ECCO is larger than that in SCCO, implying that the effects of antiferromagnetism in ECCO are stronger than that in SCCO. Also, we quantitatively confirmed the effects of antiferromagnetism by the band dispersions to tight-bindi…
Recent experiments on Pr$_{0.89}$LaCe$_{0.11}$CuO$_4$ observe an anisotropic spin-correlation gap and a nonmonotonic superconducting (SC) gap, which we analyze within the framework of a $t-t^{\prime}-t^{\prime\prime}-t^{\prime\prime\prime}-t^{iv}-U$ model with a $d_{x^2-y^2}$ pairing interaction including a third harmonic contribution. By introducing a realistic broadening of the quasiparticle spectrum to reflect small-angle scattering, our computations explain the experimental observations, especially the presence of a maximum in the leading edge gap in the vicinity of the hot-spots. Our anal…
It is shown theoretically that, in the superconducting state with $d_{x^2-y^2}$-pairing, a strong Pauli paramagnetic depairing (PD) induces not only the modulated Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state but also an incommensurate antiferromagnetic (AFM) (or spin-density wave) ordering with ${\bf Q}$-vector nearly paralle to a gap node. This intrinsic mechanism of field-induced coexistence of the $d$-wave superconducting and AFM orders does not need to assume a pair-density wave but is enhanced by the FFLO ordering. It is argued that this is the common origin of {\it both}…
Neutron and x-ray diffraction studies show that the simultaneous first-order transition to an orthorhombic and antiferromagnetic (AFM) ordered state in BaFe2As2 splits into two transitions with Co doping. For Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS = 60 K, followed by a second-order transition to AFM order at TN = 47 K. Superconductivity (SC) occurs in the orthorhombic state below TC = 15 K and coexists with AFM. Below TC, the static Fe moment is reduced and a 4 meV spin gap develops indicating competition between coexisting SC and AFM order.
The evolution of the electronic state through the reduction annealing has been investigated in electron-doped Pr_1.3-x_La_0.7_Ce_x_CuO_4+delta_ (x=0.10) single crystals with the so-called T' structure. From the ab plane and c axis electrical resistivity measurements in magnetic fields, it has been found that, through the reduction annealing, the strongly localized state of carriers accompanied by the antiferromagnetic (AF) pseudogap in the as-grown crystal changes to a metallic state bringing about the Kondo effect without AF pseudogap and to a superconducting state. These results are able to …
The competition between antiferromagnetism and the $d+id$ superconducting state is studied in a model with near and next near neighbour interactions in the absence of any on-site repulsion. A mean field study shows that it is possible to have simultaneous occurrence of an antiferromagnetic and a singlet $d+id$ superconducting state in this model. In addition, such a coexistence generates a triplet $d+id$ superconducting order parameter with centre of mass momentum $Q=(π, π)$ {\it dynamically} having the same orbital symmetry as the singlet superconductor. Inclusion of next nearest neighbour ho…
In several members of the pnictide materials, spin density wave order coexists with superconductivity over a range of dopings and temperature. In this paper we show that odd-frequency superconductivity emerges on the edges of pnictides in such a coexistence phase. In particular, the breaking of spin-rotation symmetry by SDW and translation symmetry by the edge can lead to the development of odd-frequency spin-triplet Cooper pairing on edges of superconducting pnictide samples. In this case, the odd-frequency pairing has even parity components, which are immune to disorder. Our results show tha…
We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa$_2$Ca$_4$Cu$_5$O$_y$ (Hg-1245) with $T_c=108$ K, which is composed of two types of CuO$_2$ planes in a unit cell; three inner planes (IP's) and two outer planes (OP's). The Cu-NMR study has revealed that the optimallydoped OP undergoes a superconducting (SC) transition at $T_c=108$ K, whereas the three underdoped IP's do an antiferromagnetic (AF) transition below $T_N\sim$ 60 K with the Cu moments of $\sim (0.3-0.4)μ_B$. Thus bulk superconductivity with a high value of $T_c=108$ K and a static AF …
We have synthesized polycrystalline samples and single crystals of Fe(Te1-xSx)y, and characterized their properties. Our results show that the solid solution of S in this system is limited, < 30%. We observed superconductivity at ~ 9 K in both polycrystalline samples Fe(Te1-xSx)y with 0< x <= 0.3 and 0.86 <= y <= 1.0, and single crystals with the composition Fe(Te0.9S0.1)0.91, consistent with the recent report of Tc ~ 10 K superconductivity in the FeTe1-xSx polycrystalline samples with x = 0.1 and 0.2. Furthermore, our systematic studies show that the superconducting properties of this system …
Numerical studies of the two-dimensional d-p model using the Gutzwiller ansatz have exhibited the incommensurate antiferromagnetic state coexisting with superconductivity in the under- and lightly doped regions. Our results are based on the variational Monte Carlo method for the three-band Hubbard model with d and p orbitals. We obtained the finite superconducting condensation energy for the coexistent sate at the doping rate $x=1/8$, 1/12, and 1/16, up to the systems of 256 unit cells with 768 atoms (oxygen and copper atoms). The phase diagram for the hole-doped case is consistent with recent…
We report a site selective Cu-NMR study on under-doped Hg-based five-layered high-$T_{\rm c}$ cuprate HgBa2Ca4Cu5Oy with a Tc=72 K. Antiferromagnetism (AF) has been found to take place at TN=290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69uB at three inner planes (IP's). This value is comparable to the values reported for non-doped cuprates, suggesting that the IP may be in a nearly non-doped regime. Most surprisingly, the AF order is also detected with M(OP)=0.1uB even at two outer planes (OP's) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc …
Proximity to a Mott insulating phase is likely to be an important physical ingredient of a theory that aims to describe high-temperature superconductivity in the cuprates. Quantum cluster methods are well suited to describe the Mott phase. Hence, as a step towards a quantitative theory of the competition between antiferromagnetism (AFM) and d-wave superconductivity (SC) in the cuprates, we use Cellular Dynamical Mean Field Theory to compute zero temperature properties of the two-dimensional square lattice Hubbard model. The d-wave order parameter is found to scale like the superexchange coupli…
We analyze the possible existence of a superconducting state in a background with long-range antiferromagnetism. We consider a generalized Hubbard model with nearest-neighbor correlated hopping in a square lattice. Near half filling, the model exhibits a d-wave-Bardeen-Cooper-Schrieffer (BCS) solution in the paramagnetic state. The superconducting solution would be enhanced by the antiferromagnetic background if the contribution of triplet pairs with d-wave symmetry and total momentum (pi, pi) could be neglected. However, we find that due to their contribution, the coexistence of superconducti…
We investigate the pairing in iron pnictides in the coexistence phase, which displays both superconducting and antiferromagnetic orders. By solving the pairing problem on the Fermi surface reconstructed by long-range magnetic order, we find that the pairing interaction necessarily becomes angle-dependent, even if it was isotropic in the paramagnetic phase, which results in an angular variation of the superconducting gap along the Fermi surfaces. We find that the gap has no nodes for a small antiferromagnetic order parameter M, but may develop accidental nodes for intermediate values of M, when…
We have succeeded in growing single crystals of the heavy-fermion superconductor CeCo(In1-xZnx)5 with x<=0.07. Measurements of specific heat, electrical resistivity, dc magnetization and ac susceptibility revealed that the superconducting (SC) transition temperature Tc decreases from 2.25 K (x=0) to 1.8 K (x=0.05) by doping Zn into CeCoIn5. Furthermore, these measurements indicate a development of a new ordered phase below T_o ~ 2.2 K for x=>0.05, characterized by the reduced magnetization and electrical resistivity in the ordered phase, and the enhancement of specific heat at T_o. This phase …
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