Co-based shandite Co$_3$Sn$_2$S$_2$ is a representative example of magnetic Weyl semimetals showing rich transport phenomena. We thoroughly investigate magnetic and transport properties of hole-doped shandites Co$_3$In$_x$Sn$_{2-x}$S$_2$ by first-principles calculations. The calculations reproduce nonlinear reduction of anomalous Hall conductivity with doping In for Co$_3$Sn$_2$S$_2$, as reported in experiments, against the linearly decreased ferromagnetic moment within virtual crystal approximation. We show that a drastic change in the band parity character of Fermi surfaces, attributed to the nodal rings lifted energetically with In-doping, leads to strong enhancement of anomalous Nernst conductivity with reversing its sign in Co$_3$In$_x$Sn$_{2-x}$S$_2$.
Electronic and structural properties of Rh- and Pd-based kagome layered shandites from first principles
Niels Bohr Institute, University of Copenhagen（哥本哈根大学尼尔斯·玻尔研究所）；Department of Chemical Engineering and Materials Science, University of Minnesota（明尼苏达大学化学工程与材料科学系）
$T_{3}$Pb$_{2}Ch_{2}$ ($T$=Pd,Pt and $Ch$=S,Se) with transition metal kagome net: Dynamical properties, phonon nodal line, phonon surface states, and chiral phonons
Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+δ}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferrom…
Pyrochlore iridates have recently attracted growing interest in condensed matter physics because of their potential for realizing new topological states. In order to achieve such quantum states, it is essential to understand themagnetic properties of these compounds, as their electronic structures are strongly coupled with theirmagnetic ground states. In this work, we report a systematic study of the magnetic properties of pyrochlore Y2Ir2O7 and its hole-doped compounds by performing magnetic, electron spin resonance, electrical transport, and x-ray photoelectron spectroscopy (XPS) measurement…
Herbertsmithite and Zn-doped barlowite are two compounds for experimental realization of twodimensional gapped kagome spin liquid. Theoretically, it has been proposed that charge doping a quantum spin liquid gives rise to exotic metallic states, such as high-temperature superconductivity. However, one recent experiment about herbertsmithite with successful Li-doping shows surprisingly the insulating state even under the heavy doped scenario, which can hardly be explained by many-body physics. Using first-principles calculation, we performed a comprehensive study about the Li intercalated dopin…
We have studied the magnetic and transport behavior of doped La0.65 A0.35 Mn0.95Fe0.05 O3 (A= Ca, Sr, Pb, Ba) compounds. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition except the Pb doped sample which is insulating and ferromagnetic in the entire temperature range. The simultaneous occurrence of ferromagnetism and insulating behavior in Pb doped compound is most likely due to the presence of FM clusters separated by Fe and Mn ions that are coupled AFM and hence prevent the current from crossing the inter-domain region. The magnetization and Tc are decreased …
We report the first Raman scattering measurements of BaC$_6$, SrC$_6$, and YbC$_6$, which along with new data on CaC$_6$, permits a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC$_6$, SrC$_6$, YbC$_6$, and CaC$_6$. This is due to increasing charge in the $π^*$ electronic band. Electron-phonon interaction effects between the in-plane carbon modes at $\approx1500$\invcm and the $π^*$ electrons cause a strong non-adiaba…
A comprehensive study of impurity-induced magnetism in nonmagnetically (Mg${}^{2+}$) and magnetically (Co${}^{2+}$) doped PbNi${}_{2}$V${}_{2}$O${}_{8}$ compounds is given, using both macroscopic dc susceptibility and local-probe electron spin resonance (ESR) techniques. Magnetic coupling between impurity-liberated spins is estimated from a linewidth of low-temperature ESR signal in Mg-doped samples. In addition, in the case of magnetic cobalt dopants the impurity-host magnetic exchange is evaluated from the Co-induced contribution to the linewidth in the paramagnetic phase. The experimentally…
We present a rationalization of the Raman spectra of orthorhombic and rhombohedral, stoichiometric and doped, manganese perovskites. In particular we study RMnO3 (R= La, Pr, Nd, Tb, Ho, Er, Y and Ca) and the different phases of Ca or Sr doped RMnO3 compounds as well as cation deficient RMnO3. The spectra of manganites can be understood as combinations of two kinds of spectra corresponding to two structural configurations of MnO6 octahedra and independently of the average structure obtained by diffraction techniques. The main peaks of compounds with regular MnO6 octahedra, as CaMnO3, highly Ca …
In this editorial, we first give a brief survey of the field of iron-selenide superconductivity, both in the case of bulk FeSe characterized by the co-existence of superconductivity and nematic order, and in the case of electron-doped FeSe characterized by high-temperature superconductivity. We next review the 8 contributions to the Frontiers in Physics research topic "High-Tc Superconductivity in Electron-Doped Iron Selenide and Related Compounds".
We report the thermoelectric (TE) properties of Na$_{0.74}$Co$_{0.95}$Nb$_{0.05}$O$_{2}$ in the temperature range $300-1200$ K, as a potential candidate for p-type thermoelectric material. The experimental values of Seebeck coefficient (S) are $ \sim $ $82-121$ $ μ$V/K measured in the temperature range $300-620$ K. The positive values of S in the entire temperature range indicates p-type behaviour of the compound. At 300 K the experimental value of thermal conductivity ($ κ$) is $ \sim $ 1.88 W/m-K that increases up to $ \sim $ 420 K, then decreases till 620 K with corresponding value $ \sim $…
High temperature superconductivity was achieved by introducing holes in a parent compound consisting of copper oxide layers separated by spacer layers. It is possible to dope some of the parent compounds with electrons, and their physical properties are bearing some similarities but also significant differences from the hole doped counterparts. Here, we use a recently developed first principles method, to study the electron doped cuprates and elucidate the deep physical reasons why their behavior is so different than the hole doped materials. We find that electron doped compounds are Slater in…
The mixed-valent multiferroic compound CaMn7O12 is studied for its magnetic and electric properties. The compound undergoes magnetic ordering below 90 K with a helimagnetic structure followed by a low temperature magnetic anomaly observed around 43 K. The present study shows that the magnetic anomaly at 43 K is associated with thermal hysteresis indicating first order nature of the transition. The compound also shows field-cooled magnetic memory and relaxation below 43 K, although no zero-field-cooled memory is present. Clear magnetic hysteresis loop is present in the magnetization versus fiel…
Computational studies show the generation of large coexisting metallic and insulating clusters with equal electronic density in models for manganites. The clusters are induced by disorder on exchange and hopping amplitudes near first-order transitions of the non-disordered strongly coupled system. The random-field Ising model is used to explain the qualitative aspects of our results. Percolative characteristics are natural in this context. Our results explain the recently experimentally discovered micrometer size inhomogeneities in manganites. The conclusions are general and apply to a variety…
The quantum critical point (QCP) in the archetypical heavy-fermion compound CeCu$_6$ doped by Au is described, accounting for the localized $4f$-electron of Ce, using realistic electronic structure calculations combined with dynamical mean-field theory (DMFT). Magnetism trends in Ce(Cu$_{1-ε}$Au$_ε$)$_6$ are compared with those in Co-doped CeCu$_{5}$, which resides on the non-ferromagnetic side of the composition space of one of the earliest rare-earth permanent magnet compounds, Ce(Co,Cu)$_5$. The construction of a realistic Doniach phase diagram shows that the system crosses over a magnetic …
Thermoelectric properties of the system La$_2$NiO$_{4+δ}$ have been studied ab initio. Large Seebeck coefficient values are predicted for the parent compound, and to some extent remain in the hole-doped metallic phase, accompanied of an increase in the conductivity. This system, due to its layered structure would be a suitable candidate for an improvement of its thermoelectric figure of merit by nanostructurization in thin films, that has already been shown to increase the electrical conductivity ($σ$). Our calculations show that in the region around La$_2$NiO$_{4.05}$ the system has a large t…
We successfully synthesized the nickel-based compound GdONiBi with superconducting transition temperature about 4.5 K. By partially substituting the element Gd with Sr to introduce holes into the material, we got new superconductor Gd0.9Sr0.1ONiBi with critical temperature about 4.7 K. The normal state resistivity in nickel-based samples shows a metallic behavior. The magnetoresistance measurements show a different behavior compared to those in iron-based compounds which indicates that the mechanism in the two kinds of superconductors maybe different.
ESR-study of the Ni-doped spin-Peierls compound CuGeO3 has been performed in the frequency range 9-75 GHz. At low temperatures the g-factor is smaller than the value expected for Cu- and Ni-ions. This anomaly is explained by the formation of magnetic clusters around the Ni-ions within a nonmagnetic spin-Peierls matrix. The transition into the AFM-state detected earlier by neutron scattering for doped samples was studied by means of ESR. For x=0.032 a gap in the magnetic resonance spectrum is found below the Neel temperature and the spectrum is well described by the theory of antiferromagnetic …
LaCuO_2.5 is a system of coupled, two-chain, cuprate ladders which may be doped systematically by Sr substitution. Motivated by the recent synthesis of single crystals, we investigate theoretically the possibility of superconductivity in this compound. We use a model of spin fluctuation-mediated superconductivity, where the pairing potential is strongly peaked at πin the ladder direction. We solve the coupled gap equations on the bonding and antibonding ladder bands to find superconducting solutions across the range of doping, and discuss their relevance to the real material.
Motivated by magnetization studies of the "telephon number compound" Sr_14Cu_24O_41 we investigate doped Heisenberg spin rings by means of complete numerical diagonalization of a Heisenberg Hamiltonian that depends parametrically on hole positions. A comparison with experimental magnetization data reveals rather accurate information about the screened electrostatic interaction between the charged holes on the ring which appears to be astonishingly strong.
We elucidate the existing controversies in the newly discovered K-doped iron selenide (KxFe2-ySe2-z) superconductors. The stoichiometric KFe2Se2 with \surd2\times\surd2 charge ordering was identified as the parent compound of KxFe2-ySe2-z superconductor using scanning tunneling microscopy and spectroscopy. The superconductivity is induced in KFe2Se2 by either Se vacancies or interacting with the anti-ferromagnetic K2Fe4Se5 compound. Totally four phases were found to exist in KxFe2-ySe2-z: parent compound KFe2Se2, superconducting KFe2Se2 with \surd2\times\surd5 charge ordering, superconducting …
The phase diagram of doped manganate compounds La_{1-x}A_xMnO_3 (with divalent A) is studied. We analyze an extension of the double exchange model using the Schwinger boson formalism. Earlier work by de Gennes on the existence of a canted phase is reproduced, although this phase is shown to be unstable towards phase separation in a broad regime of physical interest. We numerically solve the model's mean field equations and plot its phase diagram.
We perform spin-polarized two-dimensional angular correlation of annihilation radiation (2D-ACAR) calculations for the recently predicted ZrCo$_2$Sn-Weyl Heusler compound within the density functional theory using the generalized gradient approximation (GGA) and its extension GGA+U. We confirm that within the GGA+U method, a pair of Weyl-points are revealed, and that by doping with Niobium, for the composition Nb$_{0.3}$Zr$_{0.7}$Co$_2$Sn, the Weyl points are reaching the Fermi level. Our 2D-ACAR results indicate the existence of the Weyl points, however, within the present calculation, it is …
We explore mechanisms of orbital order decay in doped Mott insulators $R_{1-x}$(Sr,Ca)$_x$VO$_3$ ($R=\,$Pr,Y,La) caused by charged (Sr,Ca) defects. Our unrestricted Hartree-Fock analysis focuses on the combined effect of random, charged impurities and associated doped holes up to $x=0.5$. The study is based on a generalized multi-band Hubbard model for the relevant vanadium $t_{2g}$ electrons, and includes the long-range (i) Coulomb potentials of defects and (ii) electron-electron interactions. We show that the rotation of occupied $t_{2g}$ orbitals, induced by the electric field of defects, i…
The intermetallic compound LaAgSb2 displays two charge-density-wave (CDW) transitions, which were detected with measurements of electrical resistivity (rho), magnetic susceptibility, and X-ray scattering; the upper transition takes place at T1 approx. 210 K, and it is accompanied by a large anomaly in rho(T), whereas the lower transition is marked by a much more subtle anomaly at T2 approx. 185 K. We studied the effect of hydrostatic pressure (P) on the formation of the upper CDW state in pure and doped La1-xRxAgSb2 (R = Ce, Nd) compounds, by means of measurements of rho(T) for P < 23 kbar. We…
We report on the crystal growth of the quasi one-dimensional quantum spin chain compound SrCuO$_{2}$ and its doped variants containing magnetic cobalt (0.25%, 0.5%, 1% and 2.5%) and non-magnetic zinc (0.5\% and 1\%) impurities. Crystals are grown using the travelling solvent floating zone (TSFZ) method in a four-mirror optical furnace. Some crucial factors, key to the stability of a TSFZ process, including the choice of solvent composition and its associated melting behavior, are discussed in detail. The grown crystals were characterized using x-ray powder diffraction, scanning electron micros…
By measuring the nuclear magnetic resonance (NMR) 1/T_1 relaxation rate in the Br (bond) doped DTN compound, Ni(Cl(1-x)Br_x)2-4SC(NH2)2 (DTNX), we show that the low-energy spin dynamics of its high magnetic field "Bose-glass" regime is dominated by a strong peak of spin fluctuations found at the nearly doping-independent position H* = 13.6 T. From its temperature and field dependence we conclude that this corresponds to a level crossing of the energy levels related to the doping-induced impurity states. Observation of the local NMR signal from the spin adjacent to the doped Br allowed us to fu…
We report on the anomalous magnetization reversal under hydrostatic pressure in the lightly electron-doped manganite compound (Ca$_{1-x}$Sr$_{x}$)Mn$_{0.95}$Sb$_{0.05}$O$_{3}$ with its fixed carrier content. In a weakly magnetic field cooled measurement, diamagnetic magnetization is observed for $x\leq 15\%$, which changes to positive values for $x>15\%$. However, on an application of pressure on the samples with $x=16\%$ and 17$\%$, magnetization reverses sign and diamagnetism is restored. We present a magnetic phase diagram as a function of Sr concentration, under both ambient and hydrostati…
Substances such as the ``telephone number compound'' Sr14Cu24O41 are intrinsically hole-doped. The involved interplay of spin and charge dynamics is a challenge for theory. In this article we propose to describe hole-doped Heisenberg spin rings by means of complete numerical diagonalization of a Heisenberg Hamiltonian that depends parametrically on hole positions and includes the screened Coulomb interaction among the holes. It is demonstrated that key observables like magnetic susceptibility, specific heat, and inelastic neutron scattering cross section depend sensitively on the dielectric co…
The electrical resistivity, Hall coefficient, and thermopower as functions of temperature are reported for lightly electron-doped Ca(1-x)La(x)MnO(3)(0 <= x <= 0.10). Unlike the case of hole-doped ferromagnetic manganites, the magnitude and temperature dependence of the Hall mobility for these compounds is found to be inconsistent with small-polaron theory. The transport data are better described by the Feynman polaron theory and imply intermediate coupling (alpha \~ 5.4) with a band effective mass, m*~4.3 m_0, and a polaron mass, m_p ~ 10 m_0.
A ferromagnetic semiconductor, rutile (Ti,Co)O2, exhibits anomalous Hall effect at room temperature. Strong dependence of the anomalous Hall effect, as well as the magneto-optic response, on the carrier concentration suggests carrier induced ferromagnetism in this compound. Both ferromagnetic responses are caused by the charge carriers at the band edge of host semiconductor, indicating possibilities of spintronics devices operable at room temperature.
We present a relativistic spin-fermion model for the cuprates, in which both the charge and spin degrees of freedom are treated dynamically. The spin-charge coupling parameter is associated with the doping fraction. The model is able to account for the various phases of the cuprates and their properties, not only at low and intermediate doping but also for (highly) over-doped compounds. In particular, we acquire a qualitative understanding of high-T_c superconductivity through Bose-Einstein condensation of bound charge pairs. The mechanism that binds these pairs does not require a Fermi sea.
The Weyl semimetallic compound Eu2Ir2O7 along with its hole doped derivatives (which is achieved by substituting trivalent Eu by divalent Sr) are investigated through transport, magnetic and calorimetric studies. The metal-insulator transition (MIT) temperature is found to get substantially reduced with hole doping and for 10% Sr doping the composition is metallic down to temperature as low as 5 K. These doped compounds are found to violate the Mott-Ioffe-Regel condition for minimum electrical conductivity and show distinct signature of non-Fermi liquid behavior at low temperature. The MIT in …
We report the magnetic properties in the Co- and Zn-doped spin-1/2 honeycomb-lattice compound In$_3$Cu$_2$VO$_9$. The magnetic susceptibility and specific heat experiments show no long-range ordering down to 2 K in In$_3$Cu$_2$VO$_9$. In the low temperature range, approximately T$^{2}$-dependent magnetic specific heat and linearly T-dependent spin susceptibility were observed, suggesting a spin liquid candidate with a S = 1/2 honeycomb lattice. When Cu$^{2+}$ ions are partially substituted by Co$^{2+}$ ions, both impurity potential scattering and magnetic impurity scattering induced by magneti…
In hole-doped high-Tc superconductors, thermal conductivity increases drastically just below Tc, which has been considered as a hallmark of a nodal gap. In contrast, such a coherence peak in thermal conductivity is not visible in electron-doped compounds, which may indicate a full-gap state such as a (d+is)-wave state. To settle this problem, we study the thermal conductivity in the Hubbard model using the fluctuation-exchange (FLEX) approximation, which predicts that the nodal d-wave state is realized in both hole-doped and electron-doped compounds. The contrasting behavior of thermal conduct…
The nickelate compound Y_{2}BaNiO_{5} is a spin-1 Haldane-gap antiferromagnet. The compound is doped with holes on replacing the off-chain Y^{3+} ions by Ca^{2+} ions. Inelastic neutron scattering (INS) experiments reveal the existence of sub-gap states on doping. A recent INS experiment provides evidence for an incommensurate double-peaked structure factor S(q) corresponding to the sub-gap states. In this paper, we formulate a microscopic theory for the origin of the incommensurate peak.
Pseudogap regime for the prototype high-Tc compounds hole doped Bi2Sr2CaCu2O8-x (Bi2212) and electron doped Nd2-xCexCuO4 (NCCO) is described by means of novel generalized LDA+DMFT+Sk approach. Here conventional dynamical mean-field theory (DMFT) equations are supplied with additional (momentum dependent) self-energy Sk. In the present case Sk describes non-local dynamical correlations induced by short-ranged collective Heisenberg-like antiferromagnetic spin fluctuations. Material specific model parameters of two neighboring CuO2 layers of Bi2212 and single CuO2 layer of NCCO were obtained with…
We studied the relationship between the charge doping and the correlation, and its effects on the spectral function of the BaFe$_2$As$_2$ compound in the framework of the density functional theory combined with the dynamical mean field theory (DFT+DMFT). The calculated mass enhancements showed that the electronic correlation varies systematically from weak to strong when moving from the heavily electron-doped regime to the heavily hole-doped one. Since the compound has a multi-orbital nature, the correlation is orbital-dependent and it increases as hole-doping increases. The Fe-3d$_{xy}$ (xy) …
We propose the parent compound of the newly discovered superconducting nickelate Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ as a self-doped Mott insulator, in which the low-density Nd-$5d$ conduction electrons couple to localized Ni-3$% d_{x^{2}-y^{2}}$ electrons to form Kondo spin singlets at low temperatures. This proposal is motivated with our analyses of the reported resistivity and Hall coefficient data in the normal state, showing logarithmic temperature dependence at low temperatures. In the strong Kondo coupling limit, we derive a generalized $t$-$J$ model with both Kondo singlets and nickel holons m…
Ce(Fe_{1-x}Ga_x)_2 compounds with x = 0, 0.01, 0.025 and 0.05 have been investigated to unravel the effect of Ga on the magnetic state of CeFe_2. For the first time, we find that the dynamic antiferromagnetic phase present in CeFe_2 gets stabilized with Ga substitution. The hysteresis loops show that while the compounds with x = 0 and 0.01 show normal behavior, the other two show multiple magnetization steps across the antiferromagnetic-ferromagnetic transition region. The virgin curve is found to lie outside the envelope curve in these two compounds, similar to the observations made in Ru and…
The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound (x=0.06) that share a remarkable similarity with those in the paramagnetic state of the parent compound (x=0). In the overdoped superconducting compound (x=0.14), magnetic excitations show a gap-like behavior, possibly related to a topological change in the hole Fermi surface (Lifshitz transition), while the imaginary part of…
We consider a class of strongly correlated Fermi systems that exhibit an interaction-induced flat band pinned to the Fermi surface, and generalize the Landau strategy to accommodate a flat band and apply the more comprehensive theory to electron systems of solids. The non-Fermi-liquid behavior that emerges is compared with relevant experimental data on heavy-fermion metals and electron-doped high-$T_c$ compounds. We elucidate how heavy-fermion metals have extremely low superconducting transition temperature $T_c$, its maximum reached in the heavy-fermion metal CeCoIn$_5$ does not exceed 2.3 K,…
We present a systematic investigation of the electrical, structural, and antiferromagnetic properties for the series of Ba(Fe$_{1-x-y}$Co$_{x}$Rh$_{y}$)$_{2}$As$_{2}$ compounds with fixed $x \approx$ 0.027 and $ 0 \leq y \leq 0.035$. We compare our results for the Co-Rh doped Ba(Fe$_{1-x-y}$Co$_{x}$Rh$_{y}$)$_{2}$As$_{2}$ compounds with the Co doped Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ compounds. We demonstrate that the electrical, structural, antiferromangetic, and superconducting properties of the Co-Rh doped compounds are similar to the properties of the Co doped compounds. We find that the…
We report on the study of insulator-to-metal transition in post-perovskite compound CaIrO3. It is discovered that a gradual chemical substitution of calcium by yttrium leads to the onset of strong metallic behavior in this compound. This observation is in stark contrast to BaIrO3, which preserves its Mott insulating behavior despite excess of charge carriers due to yttrium doping. Magnetic measurements reveal that both compounds tend to exhibit magnetic character irrespective of the chemical substitution of Ca or Ba. We analyze these unusual observations in light of recent researches that sugg…
We have studied the magnetic and transport behavior of doped La0.65 A0.35 Mn0.95Fe0.05 O3 (A= Ca, Sr, Pb, Ba) compounds. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition except the Pb doped sample which is insulating and ferromagnetic in the entire temperature range. The simultaneous occurrence of ferromagnetism and insulating behavior in Pb doped compound is most likely due to the presence of FM clusters separated by Fe and Mn ions that are coupled AFM and hence prevent the current from crossing the inter-domain region. The magnetization and Tc are decreased …
A study of an impurity driven phase-transition into a magnetically ordered state in the spin-liquid Haldane chain compound PbNi2V2O8 is presented. Both, macroscopic magnetization as well as 51V nuclear magnetic resonance (NMR) measurements reveal that the spin nature of dopants has a crucial role in determining the stability of the induced long-range magnetic order. In the case of nonmagnetic (Mg2+) doping on Ni2+ spin sites (S=1) a metamagnetic transition is observed in relatively low magnetic fields. On the other hand, the magnetic order in magnetically (Co2+) doped compounds survives at muc…
Although the vast majority of high-$T_c$ cuprate superconductors are hole-doped, a small family of electron-doped compounds exists. Under investigated until recently, there has been tremendous recent progress in their characterization. A consistent view is being reached on a number of formerly contentious issues, such as their order parameter symmetry, phase diagram, and normal state electronic structure. Many other aspects have been revealed exhibiting both their similarities and differences with the hole-doped compounds. This review summarizes the current experimental status of these materia…
We present a combined high-resolution x-ray diffraction and x-ray resonant magnetic scattering (XRMS) study of as-grown BaFe2As2. The structural/magnetic transitions must be described as a two-step process. At T_S = 134.5 K we observe the onset of a second-order structural transition from the high-temperature paramagnetic tetragonal structure to a paramagnetic orthorhombic phase, followed by a discontinuous step in the structural order parameter that is coincident with a first-order antiferromagnetic (AFM) transition at T_N = 133.75 K. These data, together with detailed high-resolution x-ray s…
Over the last years, superconductivity has been discovered in several families of iron-based compounds. Despite intense research, even basic electronic properties of these materials, such as Fermi surfaces, effective electron masses, or orbital characters are still subject to debate. Here, we address an issue that has not been considered before, namely the consequences of dynamical screening of the Coulomb interactions among Fe-d electrons. We demonstrate its importance not only for correlation satellites seen in photoemission spectroscopy, but also for the low-energy electronic structure. Fro…
Based on the Zhang-Rice singlet picture, it is argued that the half-skyrmion is created by the doped hole in the single hole doped high-T_c cuprates with N'eel ordering. The spin configuration around the Zhang-Rice singlet, which has the form of superposition of the two different d-orbital hole spin states, is studied within the non-linear σmodel and the CP^1 model. The spin configurations associated with each hole spin state are obtained, and we find that the superposition of these spin configuration turns out to be the half-skyrmion that is characterized by a half of the topological charge. …
Possible superconductivity in recently discovered (Tl,K)Fe$_x$Se$_2$ compounds is studied from the viewpoint of doped Mott insulator. The Mott insulating phase is examined to be preferred in the parent compound at $x=1.5$ due to the presence of Fe vacancies. Partial filling of vacancies at the Fe-sites introduces electron carriers and leads to electron doped superconductivity. By using a two-orbital Hubbard model in the strong coupling limit, we find that the s-wave pairing is more favorable at small Hund's coupling, and d$_{x^2-y^2}$ wave pairing is more favorable at large Hund's coupling.
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 have studied selected rare earth doped and transition metal doped CeFe2 compounds by examining their structural, magnetic and magneto-thermal properties. With substitution of Ce by 5 and 10% Gd and 10% Ho, the Curie temperature can be tuned to the range of 267-318 K. Localization of Ce 4f electronic state with rare earth substitutions is attributed for the enhancement of Curie temperature. On the other hand, with Ga and Al substitution at the Fe site, system undergoes paramagnetic to ferromagnetic transition and then to an antiferromagnetic phase on cooling. The magnetocaloric effect across…
We have performed x-ray scattering measurements on single crystals of the doped spin-Peierls compound Ti(1-x)Sc(x)OCl (x = 0, 0.01, 0.03). These measurements reveal that the presence of non-magnetic dopants has a profound effect on the unconventional spin-Peierls behavior of this system, even at concentrations as low as 1%. Sc-doping suppresses commensurate fluctuations in the pseudogap and incommensurate spin-Peierls phases of TiOCl, and prevents the formation of a long-range ordered spin-Peierls state. Broad incommensurate scattering develops in the doped compounds near Tc2 ~ 93 K, and persi…
The mixing of orbital and spin character in the wave functions of the $5d$ iridates has led to predictions of strong couplings among their lattice, electronic and magnetic degrees of freedom. As well as realizing a novel spin-orbit assisted Mott-insulating ground state, the perovskite iridate Sr$_{2}$IrO$_{4}$ has strong similarities with the cuprate La$_{2}$CuO$_{4}$, which on doping hosts a charge-density wave that appears intimately connected to high-temperature superconductivity. These phenomena can be sensitively probed through momentum-resolved measurements of the lattice dynamics, made …
We present a systematic investigation of the antiferromagnetic ordering and structural distortion for the series of Ba(Fe{1-x}Ru{x})2As2 compounds (0 <= x <= 0.246). Neutron and x-ray diffraction measurements demonstrate that, unlike for the electron-doped compounds, the structural and magnetic transitions remain coincident in temperature. Both the magnetic and structural transitions are gradually suppressed with increased Ru concentration and coexist with superconductivity. For samples that are superconducting, we find strong competition between superconductivity, the antiferromagnetic orderi…
