The electronic phase diagram of the LaO1-xFxFeAs superconductor
H. Luetkens, H. -H. Klauss, M. Kraken, F. J. Litterst, T. Dellmann, R. Klingeler, C. Hess, R. Khasanov, A. Amato, C. Baines, M. Kosmala, O. J. Schumann, M. Brad… 展开作者
H. Luetkens, H. -H. Klauss, M. Kraken, F. J. Litterst, T. Dellmann, R. Klingeler, C. Hess, R. Khasanov, A. Amato, C. Baines, M. Kosmala, O. J. Schumann, M. Braden, J. Hamann-Borrero, N. Leps, A. Kondrat, G. Behr, J. Werner, B. Buechner
备注：
25 pages, 7 figures
发表信息：
Nature Materials Letter April 2008, published online 22 February 2008
The competition of magnetic order and superconductivity is a key element in the physics of all unconventional superconductors, e.g. in high-transition-temperature cuprates 1, heavy fermions 2 and organic superconductors3. Here superconductivity is often found close to a quantum critical point where long-range antiferromagnetic order is gradually suppressed as a function of a control parameter, e.g. charge carrier doping or pressure. It is believed that dynamic spin fluctuations associated with this quantum critical behaviour are crucial for the mechanism of superconductivity. Recently high-temperature superconductivity has been discovered in iron-pnictides providing a new class of unconventional superconductors4,5,6. Similar to other unconventional superconductors the parent compounds of the pnictides exhibit a magnetic ground state7,8 and superconductivity is induced upon charge carrier doping. In this Letter the structural and electronic phase diagram is investigated by means of x-ray scattering, MuSR and Moessbauer spectroscopy on the series LaO1-xFxFeAs. We find a discontinuous first-order-like change of the Neel temperature, the superconducting transition temperature and of the respective order parameters. Our results strongly question the relevance of quantum critical behaviour in ironpnictides and prove a strong coupling of the structural orthorhombic distortion and the magnetic order both disappearing at the phase boundary to the superconducting state.
We report on the synthesis and structural/ microstructural studies of iron based fluorine doped LaOFeAs superconductors. We have successfully synthesized fluorine doped superconducting LaO1-xFxFeAs materials by choosing lower temperature (1150^0C) and longer synthesis duration (60 hours) as compared to the standard values of these employed in the pioneering first contribution [Kamihara et al 2008 J Am Chem Soc 130 3296]. Decrease of lattice parameters as determined by x-ray diffraction confirm the substitution of fluorine. The superconducting transition temperature is 27.5 K which is observed …
A safe, simple and easily scaleable one-step sintering method is proposed to fabricate newly discovered superconductors of SmO1-xFxFeAs. Superconducting transition with the onset temperature of 54.6 K and high critical fields Hc2(0) >=200 T were confirmed in SmO1-xFxFeAs with x = 0.3. At 5 K and self field, critical current density Jc estimated from the magnetization hysteresis using the whole sample size and the average particle size reached 8.5x10^3 and 1.2x10^6 A/cm^2, respectively. Moreover, the Jc exhibited a very weak dependence on magnetic field. Microstructural characterizations reveal…
We report on the synthesis and microstructural studies of fluorine based iron doped superconductors. We have successfully synthesized the fluorine based new superconducting material LaO1-xFxFeAs by choosing comparatively lower temperature and longer synthesis duration. The superconducting transition temperature that we have achieved is 27.5 K which is observed at doping level of x=0.2. The structural microstructural characterizations have been done by employing XRD, SEM and TEM techniques. The SEM and TEM micrographs reveal the layered structure.
Polycrystalline samples of RO1-xFxFeAs (0 < x < 0.25) (R = La, Sm, Gd) were investigated by wavelength-dispersive X-ray spectroscopy (WDX) in the electron microscope to determine the composition of the samples, in particular the fluorine content. It was found that the measured fluorine content can deviate considerably from the initial weight. In the lanthanum compound LaO1-xFxFeAs, we found good agreement mainly for x > 0.05, but for x < 0.05 the fluorine hardly goes into the sample. For the samarium compound we measured less than half the fluorine in the sample as initially weighed at all flu…
We show that the zero field normal-state resistivity above Tc for various levels of electron doping-both for underdoped, optimally, and overdoped LaO1-xFxFeAs, i.e. (x=0.05-0.075 and 0.1-0.2) and underdoped SmO1-xFxFeAs (x=0.06- 0.1) members of the 1111-iron-pnictide superconductor family-can be scaled in a broad temperature range from 20 to 300 K onto a single universal curve. The scaling was performed using the energy scale Delta, the resistivity rho(Delta) and the residual resistivity rho0 as scaling parameters as well as applying a recently proposed model-independent scaling method (H. G. …
We show that the zero field normal-state resistivity above Tc for various levels of electron doping - both for LaO1-xFxFeAs (La-1111) and SmO1-xFxFeAs (Sm-1111) members of the 1111-iron-pnictide superconductor family - can be scaled in a broad temperature range from 20 to 300 K onto single curves for underdoped La-1111 (x=0.05-0.075), for optimally and overdoped La-1111 (x=0.1-0.2) and for underdoped Sm-1111 (x=0.06-0.1) compounds. The scaling was performed using the energy scale Δ, the resistivity ρ_Δ and the residual resistivity ρ_0 as scaling parameters as well as by applying a recently pro…
We study the model of a d-wave superconductor interacting with finite concentration of Anderson impurities at zero temperature. The interaction between impurity and conduction electrons is taken into account within the large-$N$ approximation. We discuss the obtained phase diagram and its dependence on the main energy scales.
The symmetry approach to the description of the (P,T) phase diagram of ferromagnet superconductors with triplet pairing is developed. Taking into account the recent experimental observations made on UCoGe it is considered the case of a crystal with orthorhombic structure and strong spin-orbital coupling. It is shown that formation of ferromagnet superconducting state from a superconducting state is inevitably accompanied by the first order type transition.
We present an resonating valence bond (RVB) theory of superconductivity for the Hubbard--Heisenberg model on an anisotropic triangular lattice. We show that these calculations are consistent with the observed phase diagram of the half-filled layered organic superconductors, such as the beta, beta', kappa and lambda phases of (BEDT-TTF)_2X [bis(ethylenedithio)tetrathiafulvalene] and (BETS)_2X [bis(ethylenedithio)tetraselenafulvalene]. We find a first order transition from a Mott insulator to a d_{x^2-y^2} superconductor with a small superfluid stiffness and a pseudogap with d_{x^2-y^2} symmetry…
We examine the effects of a phenomenological pseudogap on the T=0 K phase diagram of a high temperature superconductor within a self-consistent model which exhibits a d-wave pairing symmetry. At the mean-field level the presence of a pseudogap in the normal phase of the high temperature superconductor is proved to be essential for the existence of a metallic--like state in the density versus interaction phase diagram. In the small density limit, at high attractive interaction, bosonic--like degrees of freedom are likely to emerge. Our result should be relevant for underdoped high temperature s…
A model for high-temperature superconductors incorporating antiferromagnetism, d-wave superconductivity, and no double lattice-site occupancy can give energy surfaces exquisitely balanced between antiferromagnetic and superconducting order for specific ranges of doping and temperature. The resulting properties can reconcile a universal cuprate phase diagram with rich inhomogeneity, relate that inhomogeneity to pseudogaps, give a fundamental rationale for giant proximity effects and other emergent behavior, and provide an objective framework for separating essential from peripheral in the super…
The renormalization group approach to correlated fermions is used to determine the phase diagram of the oxide cuprates modeled by the t-t' Hubbard model at the Van Hove filling. Spin-dependent interactions give rise to instabilities corresponding to ferromagnetic, antiferromagnetic and d-wave superconducting phases. Antiferromagnetism and d-wave superconductivity arise from the same interactions, and compete in the same region of parameter space.
We study the dynamics of the Cooper pairing across the T=0 phase diagram of the two-dimensional Hubbard Model, relevant for high-temperature superconductors, using a cluster extension of dynamical mean field theory. We find that the superconducting pairing function evolves from an unconventional form in the over-doped region into a more conventional boson-mediated retarded form in the under-doped region of the phase diagram. The boson, however, promotes the rise of a pseudo-gap in the electron density of states rather than a superconducting gap as in the standard theory of superconductivity. W…
The pressure dependences of the antiferromagnetic and superconducting transition temperatures have been investigated by ac heat capacity measurement under high pressures for the heavy-fermion superconductor CePt$_3$Si without inversion symmetry in the tetragonal structure. The Néel temperature $T_{\rm N}$ = 2.2 K decreases with increasing pressure and becomes zero at the critical pressure $P_{\rm AF}$ $\simeq$ 0.6 GPa. On the other hand, the superconducting phase exists in a wider pressure region from ambient pressure to about 1.5 GPa. The pressure phase diagram of CePt$_3$Si is thus very uniq…
We have investigated correlated electronic structures and the phase diagram of electron-doped hydrocarbon molecular solids, based on the dynamical mean-field theory. We have found that the ground state of hydrocarbon-based superconductors such as electron-doped picene and coronene is a multi-band Fermi liquid, while that of non-superconducting electron-doped pentacene is a single-band Fermi liquid in the proximity of the metal-insulator transition. The size of the molecular orbital energy level splitting plays a key role in producing the superconductivity of electron-doped hydrocarbon solids. …
Single crystals of (Ca1-xLax)10(Pt3As8)(Fe2As2)5 (x = 0 to 0.182) superconductors have been grown and characterized by X-ray, microprobe, transport and thermodynamic measurements. Features in the magnetic susceptibility, specific heat and two kinks in the derivative of the electrical resistivity around 100 K in the x = 0 compound support the existence of decoupled structural and magnetic phase transitions. With La doping, the structural/magnetic phase transitions are suppressed and a half-dome of superconductivity with a maximal Tc around 26 K is observed in the temperature-concentration phase…
We propose a theoretical description of the phase diagram and physical properties in A2Fe4Se5-type (A=K, Tl) compounds based on a coexistent local moment and itinerant electron picture. Using neutron scattering and ARPES measurements to fix the general structure of the local moment and itinerant Fermi pockets, we find a superconducting (SC) regime with s-wave pairing at the M pockets and an incipient sign-change s-wave near the Gamma point, which is adjacent to an insulating state at low doping and a charge-density-wave (CDW) state at high doping. The uniform susceptibility and resistivity are…
The magnetic susceptibility of the organic superconductors $κ$-(h8 or d8-ET)$_{2}$$X$, $X =$ Cu(NCS)$_{2}$ and Cu[N(CN)$_{2}$]Br has been studied. A metallic phase below $T^{*} =$ 37 $\sim$ 38 K for $X =$ Cu[N(CN)$_{2}$]Br and 46 $\sim$ 50 K for $X =$ Cu(NCS)$_{2}$ has an anisotropic temperature dependence of the susceptibility and the charge transport. Partial charge-density-wave or charge fluctuation is expected to coexist with the metallic phase instead of the large antiferromagnetic fluctuation above $T^{*}$. The phase diagram and the superconductivity of $κ$-(ET)$_{2}$$X$ are discussed in…
The discovery of the multiple $H$-$T$ phase diagram of CeRh$_2$As$_2$ offers a new route to designing topological superconductors. Although weak-coupling theories explain the experimental phase diagram qualitatively, a quantitative discrepancy between them has discouraged conclusive interpretation. In this Letter, we thoroughly study the effect of Coulomb interaction and the phase diagrams of locally noncentrosymmetric superconductors. We reveal even-odd parity transition and the enhancement of the parity transition field in strongly correlated superconductors, and an issue of CeRh$_2$As$_2$ i…
The work by Soares et al. [Phys. Rev. B 65, 174506 (2002)] investigates the BCS-BE crossover for d-wave pairing in the 2-dimensional attractive Hubbard model. Contrary to their claims, we found that a non-pairing region does not exist in the density vs coupling phase diagram. The gap parameter at T=0, as obtained by solving analytically as well as numerically the BCS equations, is in fact finite for any non-zero density and coupling, even in the weak-coupling regime.
We report a systematic crystal growth and characterization of Ca$_{1-y}$La$_{y}$Fe$_{1-x}$Ni$_{x}$As$_{2}$, the newly discovered 112-type iron-based superconductor. After substituting Fe by a small amount of Ni, bulk superconductivity is successfully obtained in high quality single crystals sized up to 6 mm. Resistivity measurements indicate common features for transport properties in this 112-type iron pnictide, suggest strong scattering from chemical dopants. Together with the superconducting transition temperature $T_c$, and the Neel temperature $T_N$ determined by the elastic neutron scatt…
We study an extended Hubbard model with on-site repulsion and nearest neighbors attraction which tries to mimic some of the experimental features of doped cuprates in the superconducting state. We draw and discuss the phase diagram as a function of the effective interactions among electrons for a wide range of doping concentrations. We locate the region which is relevant for the cuprates setting some constraints on the parameters which may be used in this kind of effective models. We also study the effects of temperature and orthorrombicity on the symmetry and magnitude of the gap function, an…
Taking the spin-fermion model as the starting point for describing the cuprate superconductors, we obtain an effective nonlinear sigma-field hamiltonian, which takes into account the effect of doping in the system. We obtain an expression for the spin-wave velocity as a function of the chemical potential. For appropriate values of the parameters we determine the antiferromagnetic phase diagram for the YBa$_2$Cu$_3$O$_{6+x}$ compound as a function of the dopant concentration in good agreement with the experimental data. Furthermore, our approach provides a unified description for the phase diag…
Starting from a spin-fermion model for the cuprate superconductors, we obtain an effective interaction for the charge carriers by integrating out the spin degrees of freedom. Our model predicts a quantum critical point for the superconducting interaction coupling, which sets up a threshold for the onset of superconductivity in the system. We show that the physical value of this coupling is below this threshold, thus explaining why there is no superconducting phase for the undoped system. Then, by including doping, we find a dome-shaped dependence of the critical temperature as charge carriers …
We report a systematic investigation of Ba[Fe(1-x)Co(x)]2As2 based on transport and 75-As NMR measurements, and establish the electronic phase diagram. We demonstrate that doping progressively suppresses the uniform spin susceptibility and low frequency spin fluctuations. The optimum superconducting phase emerges at x_c~0.08 when the tendency toward spin ordering completely diminishes. Our findings point toward the presence of a quantum critical point near x_c between the SDW (spin density wave) and superconducting phases.
The phase diagram of superconducting copper oxides is calculated as a function of doping based on a theory of dynamic stripe induced superconductivity. The two major conclusions from the theory and the numerical analysis are that T* (the pseudogap onset) and Tc (superconductivity onset) are correlated through the pseudogap, which induces a gap in the single particle energies that persists into the superconducting state. By decreasing the doping the pseudogap Delta* increases and T* increases, but when Delta* exceeds a certain critical value the superconducting transition temperature Tc is supp…
A systematic, doping dependent interlayer tunneling spectroscopy of Bi2212 high $T_c$ superconductor is presented. An improved resolution made it possible to simultaneously trace the superconducting gap (SG) and the normal state pseudo-gap (PG) in a close vicinity of $T_c$ and to analyze closing of the PG at $T^*$. The obtained doping phase diagram exhibits a critical doping point for appearance of the PG and a characteristic crossing of the SG and the PG close to the optimal doping. This points towards coexistence of two different and competing order parameters in Bi2212. Experimental data in…
The correlation and competition between antiferromagnetism and superconductivity are one of the most fundamental issues in all of high temperature superconductors. The superconductivity in high temperature cuprate superconductors arises from suppressing an antiferromagnetic (AFM) Mott insulator phase by doping1 while that in iron-pnictide high temperature superconductors arises from AFM semimetals and can coexist with AFM orders2-9. This key difference marked in their phase diagrams has raised many intriguing debates about whether the two materials can be placed in the same category to underst…
We propose that Resistivity Curvature Mapping (RCM) based on the in-plane resistivity data is a useful way to objectively draw an electronic phase diagrams of high-T_c cuprates, where various crossovers are important. In particular, the pseudogap crossover line can be conveniently determined by RCM. We show experimental phase diagrams obtained by RCM for Bi_{2}Sr_{2-z}La_{z}CuO_{6+δ}, La_{2-x}Sr_{x}CuO_{4}, and YBa_{2}Cu_{3}O_{y}, and demonstrate the universal nature of the pseudogap crossover. Intriguingly, the electronic crossover near optimum doping depicted by RCM appears to occur rather a…
Using the fluctuation-exchange (FLEX) approximation we study an effective five-band Hubbard model for iron-pnictide superconductors obtained from the first-principles band structure. We preclude deformations of the Fermi surface due to electronic correlations by introducing a static potential, which mimics the effect of charge relaxation. Evaluating the Eliashberg equation for various dopings and interaction parameters, we find that superconductivity can sustain higher hole than electron doping. Analyzing the symmetry of the superconducting order parameter we observe clear differences between …
The role of nematic fluctuations for unconventional superconductivity has been subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal $T_C$. However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the N…
We present the results of our comprehensive investigation on the antiferromagnetic heavy-fermion superconductor Ce$_3$PtIn$_{11}$ carried out by means of electrical transport, heat capacity and ac magnetic susceptibility measurements, performed on single-crystalline specimens down to 50 mK in external magnetic fields up to 9~T. Our experimental results elucidate a complex magnetic field -- temperature phase diagram which contains both first- and second-order field-induced magnetic transitions and highlights the emergence of field stabilized phases. Remarkably, a prominent metamagnetic transiti…
We present a novel phase diagram for the antiferromagnetism and superconductivity in Ce$_2$RhIn$_8$ probed by In-NQR studies under pressure ($P$). The quasi-2D character of antiferromagnetic spin fluctuations in the paramagnetic state at $P$ = 0 evolves into a 3D character because of the suppression of antiferromagnetic order for $P > P_{\rm QCP}\sim$ 1.36 GPa (QCP: antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate $1/T_1$ measurements revealed that the superconducting order occurs in the $P$ range 1.36 -- 1.84 GPa, with maximum $T_c\sim$ 0.9 K around $P_{\rm QCP}…
We report the results of our detailed magnetotransport studies on single crystals of the antiferromagnetic heavy-fermion superconductor Ce$_3$PdIn$_{11}$. Electrical resistivity measurements, carried out in different magnetic field orientations with respect to the crystallographic axes, were mainly aimed to amass further insight about the magnetically ordered state. The results manifest a clear metamagnetic transitions in the ordered phase when the applied field is parallel to the tetragonal $c$ axis, while no similar features are seen for the transverse direction, i.e., with the field confine…
We report on the magnetic phase diagram of a hole-doped cuprate Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, which is free from buckling of CuO$_2$ planes, determined by muon spin rotation and relaxation. It is characterized by a quasi-static spin glass-like phase over a range of sodium concentration ($0.05\leq x\leq 0.12$), which is held between long range antiferromagnetic (AF) phase ($x\leq 0.02$) and superconducting phase where the system is non-magnetic for $x\geq 0.15$. The obtained phase diagram qualitatively agrees well with that commonly found for hole-doped high-\tc cuprates, strongly sugges…
We present a comprehensive study of the critical current densities and the superconducting vortex phase diagram in the stoichiometric superconductor CaKFe$_4$As$_4$ which has a critical temperature of 35 K. We performed detailed magnetization measurements both of high quality single crystals for different orientations in an applied magnetic field up to 16 T and for a powder sample. We find an extremely large critical current density, Jc, up to 10$^8$ A/cm2 for single crystals when H||(ab) at 5 K, which remains robust in fields up to 16 T, being the largest of any other iron-based superconducto…
We present the first comprehensive derivation of the intrinsic electronic phase diagram of the iron-oxypnictide superconductors in the normal state based on the analysis of the electrical resistivity $ρ$ of both LaFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$ for a wide range of doping. Our data give clear-cut evidence for unusual normal state properties in these new materials. In particular, the emergence of superconductivity at low doping levels is accompanied by distinct anomalous transport behavior in $ρ$ of the normal state which is reminiscent of the spin density wave (SDW) signature in th…
Nematicity has emerged as a key feature of cuprate superconductors, but its link to other fundamental properties such as superconductivity, charge order and the pseudogap remains unclear. Here we use measurements of transport anisotropy in YBa$_2$Cu$_3$O$_y$ to distinguish two types of nematicity. The first is associated with short-range charge-density-wave modulations in a doping region near $p = 0.12$. It is detected in the Nernst coefficient, but not in the resistivity. The second type prevails at lower doping, where there are spin modulations but no charge modulations. In this case, the on…
We report 139La, 57Fe and 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements on powders of the new LaO1-xFxFeAs superconductor for x = 0 and x = 0.1 at temperatures up to 480 K, and compare our measured NQR spectra with local density approximation (LDA) calculations. For all three nuclei in the x = 0.1 material, it is found that the local Knight shift increases monotonically with an increase in temperature, and scales with the macroscopic susceptibility, suggesting a single magnetic degree of freedom. Surprisingly, the spin lattice relaxation rates for al…
BCS theory describes the formation of Cooper pairs and their instant "Bose condensation" into a superconducting state. Helium atoms are preformed bosons and, in addition to their condensed superfluid state, can also form a quantum solid, lacking phase-coherence. Here we show that the fate of Cooper pairs can be more varied than the BCS or helium paradigms. In copper-oxide d-wave superconductors (dSC) Cooper pairs are intrinsically non-local objects, with both center-of-mass and relative motions. As doping decreases, the center-of mass fluctuations force a correlated dSC into a state with enhan…
NdFeAsO0.88F0.12 belongs to the recently discovered family of high-TC iron-based superconductors. The influence of high pressure on transport properties of this material has been studied. Contrary to La-based compounds, we did not observe a maximum in TC under pressure. Under compression, TC drops rapidly as a linear function of pressure with the slope k = -2.8 \pm 0.1 K / GPa. The extrapolated value of TC at zero pressure is about TC (0) = 51.7 \pm 0.4 K. At pressures higher than ~18.4 GPa, the superconducting state disappears at all measured temperatures. The resistance changes slope and sho…
We use heat capacity measurements as a function of field rotation to identify the nodal gap structure of CeIrIn5 at pressures to 2.05 GPa, deep inside its superconducting dome. A four-fold oscillation in the heat capacity at 0.3 K is observed for all pressures but with its sign reversed between 1.50 and 0.90 GPa. On the basis of recent theoretical models for the field-angle dependent specific heat, all data, including the sign reversal, imply a d{x^2-y^2} order parameter with nodes along [110], which constrains theoretical models of the pairing mechanism in CeIrIn5.
Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. These data delineate an $x-T$ phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe$_2$As$_2$ at 134 K apparently splits into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above $x \sim 0.025$, and appears to coexist with the broken symmet…
Second magnetization peak (SMP) in hole-doped cuprates and iron pnictide superconductors has been widely explored. However, similar feature in the family of electron-doped cuprates is not common. Here, we report the vortex dynamics study in the single crystal of an electron-doped cuprate Pr$_{0.87}$LaCe$_{0.13}$CuO$_4$ superconductor using dc magnetization measurements. A SMP feature in the isothermal $M(H)$ was observed for $H$$\parallel$$ab$-planes. On the other hand, no such feature was observed for $H$$\parallel$$c$-axis in the crystal. Using magnetic relaxation data, a detailed analysis o…
We report comprehensive resistivity measurements of single crystalline samples of the Ba(Fe(1-x)Co(x))2As2 high Tc superconductor under hydrostatic pressure up to 2.75 GPa and over a broad concentration range, 0 < x < 0.099. We show that application of pressure progressively suppresses the SDW transition temperature, T_SDW, in the underdoped regime. There is no sign of pressure-induced superconductivity in the undoped BaFe2As2 down to 1.8 K, but applied pressure dramatically enhances Tc in the underdoped regime. The effect of pressure on Tc is very small in the optimally and overdoped regimes.…
We report a genuine phase diagram for a disorder-free CuO_2 plane based on the precise evaluation of the local hole density (N_h) by site-selective Cu-NMR studies on five-layered high-Tc cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N_h=0.17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N_h< 0.17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM(Paramagnetism) phases may be present at N_h=0.15 and T=75 K, (4) Tc is maximum close to a quantum critical point (QCP) at which the AFM order collapses, s…
We report a sudden reversal in the pressure dependence of Tc in the iron-based superconductor RbFe2As2, at a critical pressure Pc = 11 kbar. Combined with our prior results on KFe2As2 and CsFe2As2, we find a universal V-shaped phase diagram for Tc vs P in these fully hole-doped 122 materials, when measured relative to the critical point (Pc, Tc). From measurements of the upper critical field Hc2(T) under pressure in KFe2As2 and RbFe2As2, we observe the same two-fold jump in (1/Tc)(-dHc2/dT) across Pc, compelling evidence for a sudden change in the structure of the superconducting gap. We argue…
We investigated the magnetic phase diagram of the first Pr-based heavy fermion superconductor PrOs4Sb12 by means of high-resolution dc magnetization measurements in low temperatures down to 0.06K. The temperature dependence of the magnetization M(T) at 0.1kOe exhibits two distinct anomalies at Tc1=1.83K and Tc2=1.65K, in agreement with the specific heat measurements at zero field. Increasing magnetic field H, both Tc1(H) and Tc2(H) move toward lower temperatures without showing a tendency of intersecting to each other. Above 10kOe, the transition at Tc2(H) appears to merge into a line of the p…
A crystallographic and magnetic phase diagram of SmFeAsO1-xFx is determined as a function of x in terms of temperature based on electrical transport and magnetization, synchrotron powder x-ray diffraction, 57Fe Mossbauer spectra (MS), and 149Sm nuclear resonant forward scattering (NRFS) measurements. MS revealed that the magnetic moments of Fe were aligned antiferromagnetically at ~144 K (TN(Fe)). The magnetic moment of Fe (MFe) is estimated to be 0.34 myuB/Fe at 4.2 K for undoped SmFeAsO; MFe is quenched in superconducting F-doped SmFeAsO. 149Sm NRFS spectra revealed that the magnetic moments…
We have studied the phase diagram of MgB$_2$ superconductor using a single crystal for ${\bf H}\parallel c$-axis. For the first time we report the existence of peak effect in the screening current in MgB$_{2}$ single crystal for ${\bf H}\parallel c$-axis. In the magnetic field regime $10<H<13.5$ kOe the local fundamental diamagnetic moment displays a very narrow diamagnetic step, with a temperature width of the same size as the zero dc-magnetic field transition. For higher field this step is transformed to a peak which is related with the peak effect in the screening current. Finally, for $H<1…
The pressure-temperature phase diagram of the heavy-electron superconductor URu2Si2 has been reinvestigated by ac-susceptibility and elastic neutron-scattering (NS) measurements performed on a small single-crystalline rod (2 mm in diameter, 6 mm in length) in a Cu-Be clamp-type high-pressure cell (P < 1.1 GPa). At ambient pressure, this sample shows the weakest antiferromagnetic (AF) Bragg reflections reported so far, corresponding to the volume-averaged staggered moment of mord ~ 0.011 mB/U. Under applied pressure, the AF scattering intensity exhibits a sharp increase at P ~ 0.7 GPa at low te…
In underdoped cuprates fluctuations of the phase of the superconducting order parameter play a role due to the small superfluid density. We consider the effects of phase fluctuations assuming the exchange of spin fluctuations to be the predominant pairing interaction. Spin fluctuations are treated in the fluctuation-exchange approximation, while phase fluctuations are included by Berezinskii-Kosterlitz-Thouless theory. We calculate the stiffness against phase fluctuations, ns(omega)/m*, as a function of doping, temperature, and frequency, taking its renormalization by phase fluctuations into a…
We report that four properties of cuprates and their evolution with doping are consequences of simply counting four-site plaquettes arising from doping: (1) the universal Tc phase diagram (superconductivity between \approx0.05 and \approx0.27 doping per CuO2 plane, and optimal Tc at \approx0.16), (2) the universal doping dependence of the room-temperature thermopower, (3) the superconducting neutron spin resonance peak (the "41 meV peak"), and (4) the dispersionless scanning tunneling conductance incommensurability. Properties (1), (3), and (4) are explained with no adjustable parameters, and …
We use a symmetry-constrained variational procedure to construct a generalization of BCS to include Cooper pairs with non-zero momentum and angular momentum. The resulting gap equations are solved at zero and finite temperature, and the doping-dependent solutions are used to construct gap and phase diagrams. We find a pseudogap terminating at a critical doping that may be interpreted in terms of both competing order and preformed pairs. The strong similarity between observation and predicted gap and phase structure suggests that this approach may provide a unified description of the complex st…
The structural flexibility at three substitution sites in LaFeAsO enabled investigation of the relation between superconductivity and structural parameters over a wide range of crystal compositions. Substitutions of Nd for La, Sb or P for As, and F or H for O were performed. All these substitutions modify the local structural parameters, while the F/H-substitution also changes band filling. It was found that the superconducting transition temperature $T_{c}$ is strongly affected by the pnictogen height $h_{Pn}$ from the Fe-plane that controls the electron correlation strength and the size of t…
We establish the superconductor-insulator phase diagram for quasi-one dimensional wires by measuring a large set of MoGe nanowires. This diagram is consistent with the Chakravarty-Schmid-Bulgadaev phase boundary, namely with the critical resistance being equal to R_Q = h/4e^2. We find that transport properties of insulating nanowires exhibit a weak Coulomb blockade behavior.
We re-examine the problem of a magnetic impurity coupled to a superconductor focusing on the role of quantum fluctuations. We study in detail, a system that consists of a one-dimensional charge conserving spin-singlet superconductor coupled to a boundary magnetic impurity. Our main finding is that quantum fluctuations lead to the destruction of Yu-Shiba-Rusinov (YSR) intra-gap bound-states in all but a narrow region of the phase diagram. We carry out our analysis in three stages, increasing the role of the quantum fluctuations at each stage. First we consider the limit of a classical impurity …
We investigate thermodynamic phases, including the phase of coexistence of superconductivity and ferromagnetism, the possible phase transitions of first and second order, and the shape of the phase diagram in mean-field approximation for a phenomenological model of spin-triplet ferromagnetic superconductors. The results are discussed in view of application to metallic ferromagnets as UGe$_2$, ZrZn$_2$, URhGe, and Fe.
The behavior of clean layered superconductors in the presence of a finite electric current and in zero-magnetic field behavior is addressed. The structure of the current temperature phase diagram and the properties of each of the four regions will be explained. We will discuss the expected current voltage and resistance characteristics of each region as well as the effects of finite size and weak disorder on the phase diagram. In addition, the reason for which a weakly non-ohmic region exists above the transition temperature will be explained.
