On the electronic structure of electron doped LaOFeAs as seen by X-ray absorption spectroscopy
T. Kroll, S. Bonhommeau, T. Kachel, H. A. Duerr, J. Werner, G. Behr, A. Koitzsch, R. Huebel, S. Leger, R. Schoenfelder, A. Ariffin, R. Manzke, F. M. F. de Groot… 展开作者
T. Kroll, S. Bonhommeau, T. Kachel, H. A. Duerr, J. Werner, G. Behr, A. Koitzsch, R. Huebel, S. Leger, R. Schoenfelder, A. Ariffin, R. Manzke, F. M. F. de Groot, J. Fink, H. Eschrig, B. Buechner, M. Knupfer
We investigated the recently found superconductor LaO_{1-x}F_xFeAs by X-ray absorption spectroscopy (XAS). From a comparison of the O K-edge with LDA calculations we find good agreement and are able to explain the structure and changes of the spectra with electron doping. An important result from this edge is a limitation of the Hubbard U to values not significantly larger than 1 eV.
From experimental Fe L_2,3-edge spectra and charge transfer multiplet calculations we gain further information on important physical values such as hopping parameters, the charge transfer energy Delta, and the on-site Hubbard U. Furthermore we find the system to be very covalent with a large amount of ligand holes. A shift in the chemical potential is visible in the O K- and Fe L_2,3-edge spectra which emphasizes the importance of band effects in these compounds.
We present first principles calculations of the atomic and electronic structure of electron-doped LaOFeAs. We find that whereas the undoped compound has an antiferromagnetic arrangement of magnetic moments at the Fe atoms, the doped system becomes non magnetic at a critical electron concentration. We have studied the electron-phonon interaction in the doped paramagnetic phase. For the $A_{1g}$ phonon, the separation between the As and Fe planes induces a non-collinear arrangement of the Fe magnetic moments. This arrangement is anti parallel for interactions mediated by As, and perpendicular fo…
We present the first experimental results of the lower critical field $H_{c1}$ of the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1}$FeAs (F-LaOFeAs) by global and local magnetization measurements. It is found that $H_{c1}$ showed an clear linear-$T$ dependence down to a temperature of 2 K, indicative of an unconventional pairing symmetry with a nodal gap function. Based on the d-wave model, we estimated a maximum gap value $Δ_0=4.0 \pm 0.6$ meV, in consistent with the recent specific heat and point-contact tunneling measurements. Taking the demagnetization factor into account, th…
Superconducting LaOFeAs family of even electron metallic systems have striking resemblance to odd electron cuprates. We suggest that this resemblance is caused by presence of two coupled 2D resonating valence bond systems in LaOFeAs . Bond charge repulsion and Hund coupling fuse the 2 species of valence bonds into closed strings with a Haldane gap, resulting in a quantum string liquid. A pair of doped holes (electrons) creates an open string and remain at the ends as holon(doublon). Charge -2e spin singlet strings condense to produce high Tc superconductivity. Higher Tc's are likely in our str…
We describe a new methodology of synthesizing the oxypnictide superconductors with potassium fluoride as a source as a source of fluorine instead of LaF3. This route also allows the substitution of potassium at lanthanum sites which leads to an increase in the upper critical field as would be expected for a multiband superconductor. We also report the highest reported Tc onset of 28.5K and highest upper critical field in these superconductors.
We synthesized Sr-doped $La_{0.85}Sr_{0.15}OFeAs$ sample with single phase, and systematically studied the effect of oxygen deficiency in the Sr-doped LaOFeAs system. It is found that substitution of Sr for La indeed induces the hole carrier evidenced by positive thermoelectric power (TEP), but no bulk superconductivity is observed. The superconductivity can be realized by annealing the as-grown sample in vacuum to produce the oxygen deficiency. With increasing the oxygen deficiency, the superconducting transition temperature ($T_c$) increases and maximum $T_c$ reaches about 26 K the same as t…
When either electron or hole doped at concentrations $x\sim 0.1$, the LaOFeAs family displays remarkably high temperature superconductivity with T$_c$ up to 55 K. In the most energetically stable $\vec Q_M = (π,π)$ antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy, and very strong magnetophonon coupling is uncovered. Doping (of either sign) beyond $x \sim 0.1$ results in Fe 3d heavy mass carriers ($m^*\sim 4-8$) with a large Fermi surface. Calculated Fe-Fe trans…
We predict atomic, electronic, and magnetic structures of a hypothetical compound LaOFeSb by first-principles density-functional calculations. It is shown that LaOFeSb prefers a stripe-type antiferromagnetic phase (i.e., spin density wave (SDW) phase) to the non-magnetic (NM) phase, with a larger Fe spin moment and greater SDW-NM energy difference than those of LaOFeAs. The SDW phase is found to favor the orthorhombic structure while the tetragonal structure is more stable in the NM phase. In the NM-phase LaOFeSb, the electronic bandwidth near the Fermi energy is reduced compared with LaOFeAs,…
A series of layered (Sr$_{1-x}$K$_x$)Fe$_2$As$_2$ compounds with nominal x=0 to 0.40 are synthesized by solid state reaction method. Similar to other parent compounds of iron-based pnictide superconductors, the pure SrFe$_2$As$_2$ shows a strong resistivity anomaly near 210 K, which was ascribed to the spin-density-wave instability. The anomaly temperature is much higher than those observed in LaOFeAs and BaFe$_2$As$_2$, the two prototype parent compounds with ZrCuSiAs- and ThCr$_2$Si$_2$-type structures. K-doping strongly suppresses this anomaly and induces superconductivity. Like in the case…
By partially substituting the tri-valence element La with di-valence element Sr in $LaOFeAs$, we introduced holes into the system. For the first time, we successfully synthesized the hole doped new superconductors $(La_{1-x}Sr_x)OFeAs$. The maximum superconducting transition temperature at about 25 K was observed at a doping level of x = 0.13. It is evidenced by Hall effect measurements that the conduction in this type of material is dominated by hole-like charge carriers, rather than electron-like ones. Together with the data of the electron doped system $La(O_{1-x}F_x)FeAs$, a generic phase …
The 57Fe Mössbauer spectroscopy was applied to an iron-based layered superconductor LaO0.89F0.11FeAs with a transition temperature of 26 K and its parent material LaOFeAs. Throughout the temperature range from 4.2 to 298 K, a singlet spectrum with no magnetic splitting was observed as a main component of each Mössbauer spectrum of the F-doped superconductor. No additional internal magnetic field was observed for the spectrum measured at 4.2 K under a magnetic field of 7 T. On the other hand, the parent LaOFeAs shows a magnetic transition at around 140 K, and this temperature is slightly lower …
We present point-contact spectroscopy data for junctions between a normal metal and the newly discovered F-doped superconductor LaO$_{0.9}$F$_{0.1-δ}$FeAs (F-LaOFeAs). A zero-bias conductance peak was observed and its shape and magnitude suggests the presence of Andreev bound states at the surface of F-LaOFeAs, which provides a possible evidence of an unconventional pairing symmetry with a nodal gap function. The maximum gap value $Δ_0\approx3.9\pm0.7$meV was determined from the measured spectra, in good agreement with the recent experiments on specific heat and lower critical field.
The occurrence of high temperature superconductivity, and the competition with magnetism, in stoichiometric and doped LaOFeAs and isostructural iron-oxypnictides is raising many fundamental questions about the electronic structure and magnetic interactions in this class of materials. There are now sufficient experimental data that it may be possible to identify the important issues whose resolution will lead to the understanding of this system. In this paper we address a number of the important issues. One important characteristic is the Fe-As distance (or more abstractly the pnictogen (Pn) he…
We study how manifestations of strong electron-phonon interaction (EPI) depend on the carrier concentration by solving the two-dimensional Holstein model for the spin-polarized fermions using an approximation free bold-line diagrammatic Monte Carlo (BDMC) method. We show that the strong EPI, obviously present at very small Fermion concentration, is masked by the Fermi blockade effects and Migdal's theorem to the extent that it manifests itself as moderate one at large carriers densities. Suppression of strong EPI fingerprints is in agreement with experimental observations in doped high tempera…
The spin dynamics of electron doped Mott insulators on a triangular lattice is studied based on the $t-J$ model. It is found that the particularly universal behaviors of integrated dynamical spin structure factor seen in the doped Mott insulators on a square lattice, are absent in the doped Mott insulators on a triangular lattice, indicating the presence of the normal state gap. As a result, the spin-lattice relaxation rate 1/$T_{1}$ divided by $T$ reduces with decreasing temperatures in the temperature region above 0.2$J$$\approx$50K, then follows a Curie-Weiss-like behavior at the temperatur…
We report synthesis of under doped non-superconducting parent compound of iron chalcogenide, i.e FeTe1-xSex (x=0.0, 0.01, 0.03, 0.05) single crystals by flux free method. These FeTe1-xSex (x=0.0, 0.01, 0.03, 0.05) single crystals are crystallized in the tetragonal structure with space group P4/nmm at room temperature. Detailed scanning electron microscopy (SEM) results showed that the crystals are formed in slab-like morphology structure. Powder X-Ray diffraction (XRD) results show that (0 0 l) peaks are shifted towards higher angel with increasing Se concentration. Coupled magnetic and struct…
We report detailed Fermi surface topology and quasiparticle dynamics of the host cobaltate Na0.7CoO2. A direct mapping of the Fermi surface is carried out by angle-resolved photoemission spectroscopy (ARPES). Fermi surface at temperature 16K is a hole-pocket centered around the Gamma-point. The highly correlated nature of the electron liquid is evidenced from strong on-site Coulomb interaction seen through a valence satellite and via renormalization of the quasiparticle dispersion. The quasiparticle exhibits spectral coherence only below 150K which includes the non-Fermi liquid regime.
Using the two-particle self-consistent approach and cluster perturbation theory for the two-dimensional t-t'-t''-U Hubbard model, we discuss weak- and strong-coupling mechanisms for the pseudogap observed in recent angle resolved photoemission spectroscopy on electron-doped cuprates. In the case of the strong-coupling mechanism, which is more relevant near half-filling, the pseudogap can be mainly driven by short range correlations near the Mott insulator. In the vicinity of optimal doping, where weak-coupling physics is more relevant, large antiferromagnetic correlation lengths, seen in neutr…
Extending our previous studies we present results for the doping-, momentum-, frequency-, and temperature- dependence of the kink-like change of the quasiparticle velocity resulting from the coupling to spin fluctuations. In the nodal direction a kink is found in both the normal and superconducting state while in the antinodal direction a kink occurs only below $T_c$ due to the opening of the superconducting gap. A pronounced kink is obtained only for hole-doped, but not for electron-doped cuprates and is characteristically different from what is expected due to electron-phonon interaction. We…
We investigate the physics of frustrated 3-leg Hubbard ladders in the band limit, when hopping across the ladder's rungs (t$_{\perp}$) is of the same order as hopping along them (t) much greater than the onsite Coulomb repulsion (U). We show that this model exhibits a striking electron-hole asymmetry close to half-filling: the hole-doped system at low temperatures develops a Resonating Valence Bond (RVB)-like d-wave gap (pseudogap close to ($π$,0)) coinciding with gapless nodal excitations (nodal liquid); in contrast, the electron-doped system is seen to develop a Mott gap at the nodes, whilst…
We have performed high resolution angle resolved photoemission (ARPES) studies on electron doped cuprate superconductors Sm2-xCexCuO4 (x=0.10, 0.15, 0.18), Nd2-xCexCuO4 (x=0.15) and Eu2-xCexCuO4 (x=0.15). Imaginary parts of the electron removal self energy show step-like features due to an electron-bosonic mode coupling. The step-like feature is seen along both nodal and anti-nodal directions but at energies of 50 and 70 meV, respectively, independent of the doping and rare earth element. Such energy scales can be understood as being due to preferential coupling to half- and full-breathing mod…
We report a systematic study of normal state Nernst effect in the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_{4-δ}$ over a wide range of doping (0.05$\leq x \leq$0.21) and temperature. At low temperatures, we observed a notable vortex Nernst signal above T$_c$ in the underdoped films, but no such normal state vortex Nernst signal is found in the overdoped region. The superconducting fluctuations in the underdoped region are most likely incoherent phase fluctuations as found in hole-doped cuprates. At high temperatures, a large normal state Nernst signal is found at dopings from slightly unde…
We study the evolution of a Mott-Hubbard insulator into a correlated metal upon doping in the two-dimensional Hubbard model using the Cellular Dynamical Mean Field Theory. Short-range spin correlations create two additional bands apart from the familiar Hubbard bands in the spectral function. Even a tiny doping into this insulator causes a jump of the Fermi energy to one of these additional bands and an immediate momentum dependent suppression of the spectral weight at this Fermi energy. The pseudogap is closely tied to the existence of these bands. This suggests a strong-coupling mechanism th…
We study the distribution of electronic spectral weight in a doped antiferromagnet with various types of charge order and compare to angle resolved photoemission experiments on lightly doped La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and electron doped Nd$_{2-x}$Ce$_x$CuO$_{4\pmδ}$. Calculations on in-phase stripe and bubble phases for the electron doped system are both in good agreement with experiment including in particular the existence of in-gap spectral weight. In addition we find that for in-phase stripes, in contrast to anti-phase stripes, the chemical potential is likely to move with doping. For …
By means of one-step model calculations the strong in-plane anisotropy seen in angle-resolved photoemission of the well-known iron pnictide prototype compounds BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ in their low-temperature antiferromagnetic phases is investigated. The fully-relativistic calculations are based on the Korringa-Kohn-Rostoker-Green function approach combined with the coherent potential approximation alloy theory to account for the disorder induced by Co substitution on Fe sites in a reliable way. The results of the calculations can be compared directly to experimental …
Samples of Nd(2-x)Ce(x)CuO(4), an electron-doped high temperature superconducting cuprate (HTSC), near optimal doping at x = 0.155 were measured via angle resolved photoemission (ARPES). We report a renormalization feature in the self energy ("kink") in the band dispersion at 50 - 60 meV present in nodal and antinodal cuts across the Fermi surface. Specifically, while the kink had previously only been seen in the antinodal region, it is now observed also in the nodal region, reminiscent of what has been observed in hole-doped cuprates.
We present optical conductivity data for Lu(Ni$_{1-x}$Co$_x$)$_2$B$_2$C over a wide range of frequencies and temperatures for x=0 and x=0.09. Both materials show evidence of being good Drude metals with the infrared data in reasonable agreement with dc resistivity measurements at low frequencies. An absorption threshold is seen at approximately 700 cm-1. In the cobalt-doped material we see a superconducting gap in the conductivity spectrum with an absorption onset at 24 +/- 2 cm-1 = 3.9$ +/- 0.4 k_BT_c suggestive of weak to moderately strong coupling. The pure material is in the clean limit an…
In a recent X-ray absorption study in boron doped diamond, Nakamura et al. have seen a well isolated narrow boron impurity band in non-superconducting samples and an additional narrow band at the chemical potential in a superconducting sample. We interpret the beautiful spectra as evidence for upper Hubbard band of a Mott insulating impurity band and an additional metallic `mid-gap band' of a conducting `self-doped' Mott insulator. This supports the basic framework of a recent theory of the present author of strongly correlated impurity band superconductivity (SCIBS) in a template of a wide-ga…
Polycrystalline Eu0.5La0.5BiS2F was synthesized by solid state reaction which crystallizes in the tetragonal CeOBiS2 structure (P4/nmm). We report here enhancement of Tc to 2.2 K in Eu0.5La0.5BiS2F (by electron doping in EuBiS2F with Tc ~ 0.3 K). Eu0.5La0.5BiS2F is semiconducting down to 3 K and an onset of superconductivity is seen at 2.2 K at ambient pressure. Upon application of pressure the Tc could be enhanced upto 10 K. Step like features are seen in the resistivity curves at intermediate pressures (0.5 - 1 GPa) which hints towards the possible existence of two phases with different Tc. …
We report results of large scale ground state density matrix renormalization group(DMRG) calculations on $t$-$t'$-$J$ cylinders with circumferences 6 and 8. We determine a rough phase diagram which appears to approximate the 2D system. While for many properties, positive and negative $t'$ values ($t'/t = \pm 0.2$) appear to correspond to electron and hole doped cuprate systems, respectively, the behavior of superconductivity itself shows an inconsistency between the model and the materials. The $t'<0$ (hole doped) region shows antiferromagnetism limited to very low doping, stripes more general…
We show here that many of the normal state properties of the cuprates can result from the new charge 2e bosonic field which we have recently (Phys. Rev. Lett. {\bf 99}, 46404 (2007) and Phys. Rev. B 77, 014512 (2008)) shown to exist in the exact low-energy theory of a doped Mott insulator. In particular, the 1) mid-infrared band including the non-vanishing of the restricted f-sum rule in the Mott insulator, 2) the $T^2$ contribution to the thermal conductivity, 3) the pseudogap, 4) the bifurcation of the electron spectrum below the chemical potential as recently seen in angle-resolved photoemi…
Charge order has emerged as a generic feature of doped cuprates, leading to important questions about its origin and its relation to superconductivity. Recent experiments on two classes of hole doped cuprates indicate a novel d-wave symmetry for the order. These were motivated by earlier spin fluctuation theoretical studies based on an expansion about hot spots in the Brillouin zone that indicated such order would be competitive with d-wave superconductivity. Here, we reexamine this problem by solving strong coupling equations in the full Brillouin zone. Our results find that bond-oriented ord…
Raman spectroscopy of a cobalt-doped iron-pnictide superconductor reveals the complex electronic structure of the superconducting state in this material. [Viewpoint on Phys. Rev. B 80, 180510 (2009), arXiv:0910.0898]
Hubbard ladders are an important stepping stone to the physics of the two-dimensional Hubbard model. While many of their properties are accessible to numerical and analytical techniques, the question of whether weakly hole-doped Hubbard ladders are dominated by superconducting or charge-density-wave correlations has so far eluded a definitive answer. In particular, previous numerical simulations of Hubbard ladders have seen a much faster decay of superconducting correlations than expected based on analytical arguments. We revisit this question using a state-of-the-art implementation of the den…
We present an Angle-Resolved PhotoElectron Spectroscopy study of the changes in the electronic structure of electron doped Ba(Fe(1-x)Co(x))2As2 across the superconducting phase transition. By changing the polarization of the incoming light, we were able to observe the opening of the gap for the inner hole pocket alpha, and to compare its behavior with the outer hole-like band beta. Measurements along high symmetry directions show that the behavior of beta is consistent with an isotropic gap opening, while slight anisotropies are detected for the inner band alpha. The implications of these resu…
The synthesis and characterization of PVA (Poly Vinyl Acetate) doped bulk MgB2 superconductor is reported here. PVA is used as a Carbon source. PVA doping effects made two distinguishable contributions: first enhancement of Jc field performance and second an increase in Hc2 value, both because of carbon incorporation into MgB2 crystal lattice. The susceptibility measurement reveals that Tc decreased from 37 to 36 K. Lattice parameter a decreased from 3.085 A to 3.081 A due to the partial substitution of Carbon at Boron site. PVA doped sample exhibited the Jc values greater than 10^5 A/cm2 at 5…
We report magnetotransport measurements and its scaling analysis for the optimally electron doped Sr(Fe${_{0.88}}$Co${_{0.12}}$)${_2}$As${_2}$ system. We pbserve that both the Kohler's and modified Kohler's scalings are violated. Interestingly, the Hall angle displays a quadratic temperature dependence similar to many cuprates and heavy fermion systems. The fact that this temperature dependence is seen in spite of the violation of modified Kohler's scaling suggests that the Hall angle and the magnetoresistance are not governed by the same scattering mechanism. We also observe a linear magnetor…
We systematically examine the asymmetry of the electronic states in the hole- and electron-doped cuprates by using the t-t'-t''-J model. Numerically exact diagonalization method is employed for a 20-site square lattice. We impose twisted boundary conditions (BC) instead of standard periodic BC. For static and dynamical correlation functions, averaging procedure over the twisted BC is used to reduce the finite-size effect. We find that antiferromagnetic spin correlation remains strong in electron doping in contrast to the case of hole doping, being similar to the case of the periodic BC. This l…
From measurements of the magnetic penetration depth, $λ(T)$, from 1.6 K to $T_c$ in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, $N_s(E)$ at T=0 by using a simple model. In this framework, the flat behavior of $λ^{-2}(T)$ at low $T$ implies $N_s(E)$ is small, possibly gapped, at low energies. The upward curvature in $λ^{-2}(T)$ near $T_c$ seen in overdoped films implies that superfluid comes from an anomalously small energy band within about $3k_BT_c$ of the Fermi surface.
High-temperature superconductivity in the copper-oxide ceramics remains an unsolved problem because we do not know what the propagating degrees of freedom are in the normal state. As a result, we do not know what are the weakly interacting degrees of freedom which pair up to form the superconducting condensate. That the electrons are not the propagating degrees of freedom in the cuprates is seen most directly from experiments that show spectral weight redistributions over all energy scales. Of course, the actual propagating degrees of freedom minimize such spectral rearrangements. This review …
We have performed angle-resolved photoemission spectroscopy (ARPES) measurements and first-principles electronic structure calculations on the electron-doped high-Tc superconductors Ln1.85Ce0.15CuO4 (Ln = Nd, Sm, and Eu). The observed Fermi surface and band dispersion show such changes that with decreasing ionic size of Ln3+, the curvature of the Fermi surface or -t'/t decreases, where t and t' are transfer integrals between the nearest-neighbor and next-nearestneighbor Cu sites, respectively. The increase of t with chemical pressure is found to be significant, which may explain the apparently…
We studied double superconducting (SC) domes in LaFeAsO1-xHx by using 75As- and 1H-nuclear magnetic resonance techniques, and unexpectedly discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric alignment of AF and SC phases in the electronic phase diagram. We demonstrated that the new AF ordering originates from the nesting between electron pockets, unlike the nesting between electron and hole pockets as seen in the majority of undoped pnictides. The new AF ordering is derived from the features common to high-Tc pnictides: howeve…
We present angle resolved photoemission (ARPES) data on Na-doped Ca$_2$CuO$_2$Cl$_2$. We demonstrate that the chemical potential shifts upon doping the system across the insulator to metal transition. The resulting low energy spectra reveal a gap structure which appears to deviate from the canonical $d_{x2-y2} ~ |cos(k_x a)-cos(k_y a)|$ form. To reconcile the measured gap structure with d-wave superconductivity one can understand the data in terms of two gaps, a very small one contributing to the nodal region and a very large one dominating the anti-nodal region. The latter is a result of the …
We present tunneling data from optimally-doped, superconducting BaFe1.86Co0.14As2 and its parent compound, BaFe2As2. In the superconductor, clear coherence-like peaks are seen across the whole field of view, and their analysis reveals nanoscale variations in the superconducting gap value, Delta. The average magnitude of 2Delta is ~7.4 kBTC, which exceeds the BCS weak coupling value for either s- or d-wave superconductivity. The characteristic length scales of the deviations from the average gap value, and of an anti-correlation discovered between the gap magnitude and the zero bias conductance…
We have investigated the charge dynamics and the nature of many-body interactions in La- and Pr- doped CaFe2As2. From the infrared part of the optical conductivity, we discover that the scattering rate of mobile carriers above 200 K exhibits saturation at the Mott-Ioffe-Regel limit of metallic transport. However, the dc resistivity continues to increase with temperature above 200 K due to the loss of Drude spectral weight. The loss of Drude spectral weight with increasing temperature is seen in a wide temperature range in the uncollapsed tetragonal phase, and this spectral weight is recovered …
We have measured intrinsic-tunneling spectra of a single CuO-layer La-doped Bi_2Sr_{2-x}La_xCuO_{6+δ} (Bi2201-La_x). Despite a difference of a factor of three in the optimal superconducting critical temperatures for Bi2201-La_{0.4} and Bi2212 (32 and 95 K, respectively) and different spectral energy scales, we find that the pseudogap vanishes at a similar characteristic temperature T*\approx 230-300K for both compounds. We find also that in Bi2201-La_x, PG humps are seen as sharp peaks and, in fact, even dominate the intrinsic spectra.
Iron arsenide superconductors based on the material LaFeAsO1-xFx are characterized by a two-dimensional Fermi surface (FS) consisting of hole and electron pockets yielding structural and antiferromagnetic transitions at x = 0. Electron doping by substituting O2- with F- suppresses these transitions and gives rise to superconductivity with a maximum Tc = 26 K at x = 0.1. However, the over-doped region cannot be accessed due to the poor solubility of F- above x = 0.2. Here we overcome this problem by doping LaFeAsO with hydrogen. We report the phase diagram of LaFeAsO1-xHx (x < 0.53) and, in add…
The interplay between unconventional superconductivity and the ordering of charge/spin density wave is one of the most vital issues in both condensed matter physics and material science. The Ti-based compound BaTi_2As_2O, which can be seen as the parent phase of superconducting BaTi_2Sb_2O, has a layered structure with a space group P4/mmm, similar to that of cuprate and iron-based superconductors. This material exhibits a charge density wave (CDW) ordering transition revealed by an anomaly at around 200 K in transport measurements. Here, we report the synthesis and systematical study of the p…
The complex phenomenology of shot quenched YBa2Cu3O6.5 with Tc=100K and 200K levels is compared with laser pulsed analogs with an eye on explaining the presumed Tc=552K of the latter. Shot quenching can produce metastable states with pronounced increases in plane metric and cell volume, accompanied by a rough doubling of Tc to a 100K level of an orthorhombic with 3-fold O coordinated chain Cu (O3 type). These states decay over a non-superconducting transition range to the conventional Tc=50K level of O24. We consider the plane expanded laser pulsed materials to contain aspects of O42 plane n-d…
Angle-resolved photoemission spectroscopy (ARPES) on optimally doped Bi2Sr2Ca0.92Y0.08Cu2O(8+delta) uncovers a coupling of the electronic bands to a 40 meV mode in an extended k-space region away from the nodal direction, leading to a new interpretation of the strong renormalization of the electronic structure seen in Bi2212. Phenomenological agreements with neutron and Raman experiments suggest that this mode is the B1g oxygen bond-buckling phonon. A theoretical calculation based on this assignment reproduces the electronic renormalization seen in the data.
Metallic Néel order stabilized by coupling between inequivalent Hubbard layers
Institute for Advanced Study in Physics, Zhejiang University（浙江大学前沿物理研究院）；Leinweber Institute for Theoretical Physics, Stanford University（斯坦福大学莱因韦伯理论物理研究所）；Department of Applied Physics, Stanford University（斯坦福大学应用物理系）；Institute for Advanced Study, Tsinghua University（清华大学高等研究院）；Department of Physics, Stanford University（斯坦福大学物理系）
We have analyzed various characteristic temperatures and energies of hole-doped high-Tc cuprates as a function of a dimensionless hole-doping concentration (pu). Entirely based on the experimental grounds we construct a unified electronic phase diagram (UEPD), where three characteristic temperatures (T*'s) and their corresponding energies (E*'s) converge as pu increases in the underdoped regime. T*'s and E*'s merge together with the Tc curve and 3.5kBTc curve at pu - 1.1 in the overdoped regime, respectively. They finally go to zero at pu - 1.3. The UEPD follows an asymmetric half-dome-shaped …
The use of simple, fast and economic experimental tools to characterize low-dimensional materials is an important step in the process of democratizing the use of such materials in laboratories around the world. Raman spectroscopy has arisen as a way of indirectly determining the thickness of nanolayers of transition metal dichalcogenides (TMDs), avoiding the use of more expensive tools such as atomic force microscopy, and it is therefore a widely used technique in the study of semiconducting TMDs. However, the study of many metallic TMDs in the limit of few atomic layers is still behind when c…
