Co doped ZnV2O4 has been investigated by Synchrotron X-ray diffraction, Magnetization measurement and Extended X-ray absorption fine structure (EXAFS) analysis. With Co doping in the Zn site the system moves towards the itinerant electron limit. From Synchrotron and magnetization measurement it is observed that there is an effect in bond lengths and lattice parameters around the magnetic transition temperature. The EXAFS study indicates that Co ion exists in the High spin state in Co doped ZnV2O4.
The magnetic nature of the B-site dopants controls the magnetic phases in B-site doped manganites RE$_{1-x}$AE$_{x}$Mn$_{1-η}$B$_η$O$_3$. Different B-site dopants of equal valence, doped into the same reference manganite, lead to different magnetic phases at low temperature, which can not be explained using the valence change scenario. We focus on trivalent B-site dopants in CE-CO-OO-I manganites at half-filling $x=0.50$ to study the role of magnetic interactions between the B-site dopants and the neighboring Mn-sites by using a two-orbital double-exchange model including super-exchange intera…
Low as well as high-temperature electron and x-ray diffraction studies have been carried out on a rare-earth free B-site disordered electron-doped manganite SrMn0.875.Mo0.125O3-δ in the temperature range of 83K to 637K. These studies reveal the occurrence of strong charge ordering (CO) at room temperature in a pseudo tetragonally distorted perovskite phase with space-group Pmmm. Non integral modulation vector of 8.95 times along [-110] indicates a charge density wave type modulation. The CO phase with basic perovskite structure Pmmm transforms to a charge disorder cubic phase through a first o…
The structure, electronic, and magnetic properties of the Mo-doped perovskite La0.7Ca0.3Mn1-xMoxO3 (x < 0.1) have been studied. A significant increase in resistivity and lattice parameters were observed with Mo doping. A marginal decrease in the Curie temperature Tc and the associated metal-insulator transition Tp were observed. Magnetization data reveal that long-range ferromagnetic ordering persists in all samples studied and the saturation moment decreases linearly as x increases. Enhancement in magnetoresistance at near Tc in the Mo-doped compounds with an optimum doping value x = 0.05 was…
The orbital order-disorder transition temperature (T_OO) versus tolerance factor (t) plot switches from monotonic to non-monotonic beyond a doping level (x) ~ 10 atom% in a family of R(1-x)A(x)MnO3 systems(R = La, Pr, Nd; A = Ca, Sr; x = 0.0-0.2). T_OO reaches maximum at a 'doping-dependent' critical tolerance factor t_C(x) (for 0.1<x<0.2)at which the orthorhombic distortion (D) also maximizes. Such an observation reflects influence of charge carriers on both 'intrinsic' octahedral-site distortion and its bias on the orbital order in doped perovskite manganites and, thus, deviation from what h…
Using the density matrix renormalization-group technique, we calculate the impurity energy levels for two different effective models of off-chain doping for quasi-one-dimensional Heisenberg chain compound Y$_2$ Ba Ni O$_5$: ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping. Thresholds of the impurity strength for the appearance of localized states are found for both models. However, the ground-state and low-energy excitations for weak impurity strength are different for these two models and the difference can be detected by experiments.
The self-interaction corrected density-functional calculations are performed for Re impurities and their pairs in silicon. Rhenium ions form in the host crystal not very tight pairs, with impurities separated by one Si atom or by a distance close to two silicon bonds. Comparison of formation energies for various pairs of substitutionals, interstitials, and mixed-site impurities favours the last type. Electron transfer from the interstitial into the substitutional impurity makes the both Re sites nonmagnetic, but the p-type and the n-type co-doping revives magnetism again, the latter more effic…
The effective mass is one of the main factors determining the Seebeck coefficient and electronic conductivity. Nb-doping increases the effective mass because of two reasons, lattice constants increase and electronic effects. In this ab-initio study the effective mass is estimated from the curvature of electronic bands and it could be clarified that the deformation of SrTiO3 crystals has a significant influence on bandgap and effective DOS and band mass, which are both in excellent agreement to experimental data. However, the electronic effect due to the e2g- band flattening near the Gamma-poin…
An effective Hamiltonian is derived in the case of the strong Hund coupling and on-site Coulomb interaction by means of a projective perturbation approach. A physical mechanism for charge ordering in half-doped manganites (R_{0.5}X_{0.5}MnO_3) is proposed. The virtual process of electron hopping results in antiferromagnetic superexchange and a repulsive interaction, which may drive electrons to form a Wigner lattice. The phase diagram of the ground state of the model is presented at half doping. In the case of formation of Wigner lattice, we prove that spins of electrons are aligned ferromagne…
Effect of Co, Ru and Cu substitution at B and B' sites on the magnetic and transport properties of LaSrCoRuO$_6$ have been investigated. All the doped compositions crystallize in the monoclinic structure in the space group $P2_1/n$ indicating a double perovskite structure. While the magnetization and conductivity increase in Co and Ru doped compounds, antiferromagnetism is seen to strengthen in the Cu doped samples. These results are explained on the basis of a competition between linear Co-O-Ru-O-Co and perpendicular Co-O-O-Co antiferromagnetic interactions and due to formation of Ru-O-Ru fer…
We discuss here a comparative study of the role of local structure and/or nature of local magnetic coupling on the electrical transport properties of Mn site substituted La-Ca-Mn-O perovskites. Particular emphasis is being paid to explore the strong correlation between the insulator-metal transition (TIM) and the residual resistivity (rhoo) upon substitution. There exists an inverse relationship between ro and TIM in the compounds under present discussion. Best fit for TIM Vs ro could be obtained for the compounds understudy to a first order exponential decay with a functional form TIM = TIMO …
Kondo insulator FeSb$_2$ with large Seebeck coefficient would have potential in thermoelectric applications in cryogenic temperature range if it had not been for large thermal conductivity $κ$. Here we studied the influence of different chemical substitutions at Fe and Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. At $5\%$ of Te doping at Sb site thermal conductivity is suppressed from $\sim 250$ W/Km in undoped sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal conductivity more slowly than Te doping, and even at 20$\%$…
Phase diagram of electron and hole-doped SrFe2As2 single crystals is investigated using Co and Mn substitution at the Fe-sites. We found that the spin-density-wave state is suppressed by both dopants, but the superconducting phase appears only for Co (electron)-doping, not for Mn (hole)-doping. Absence of the superconductivity by Mn-doping is in sharp contrast to the hole-doped system with K-substitution at the Sr sites. Distinct structural change, in particular the increase of the Fe-As distance by Mn-doping is important to have a magnetic and semiconducting ground state as confirmed by first…
The effect of quenched disorder on the multiphase competition has been investigated by examining the pressure phase diagram of half doped manganite Ln0.5B0.5MnO3 (Ln = Sm and Nd) with A-site disorders. Sm0.5Ba0.5MnO3, a spin glass insulator at ambient pressure, switches to a ferromagnetic metal with increasing pressure, followed by a rapid increase of the ferromagnetic transition temperature Tc. The rapid increase of Tc was confirmed also for Nd0.5Ba0.5MnO3. These observations indicate that the unusual suppression of the multicritical phase boundary in the A-site disordered system, previously …
We propose that a specific spatial configuration of lattice sites that energetically favor {\it 3+} or {\it 4+} Mn ions in moderately doped manganites constitutes approximately a spatially random two-energy-level system. Such an effect results in a mechanism of metal-insulator transitions that appears to be different from both the Anderson transition and the Mott-Hubbard transition. Correspondingly, a disordered Kondo lattice model is put forward, whose dynamical mean-field solution agrees reasonably with experiments.
The suppression of antiferromagnetic ordering in geometrically frustrated Hubbard models leads to a variety of exotic quantum phases including quantum spin liquids and chiral states. Here, we focus on the Hubbard model on one of the simplest frustrated lattice geometries, a triangular lattice. Motivated by the recent realization of ultracold fermionic atoms in triangular optical lattices, we study the properties of the triangular-lattice Hubbard model through a Numerical Linked-Cluster Expansion algorithm. We investigate the Mott insulator transition finding a critical interaction $U_c/t = 7.0…
We introduce the notion of superstructure Mottness to describe the Mott and Wigner-Mott transition in doped strongly correlated electron systems at commensurate filling fractions away from one electron per site. We show that superstructure Mottness emerges in an inhomogeneous electron system when the superstructure contains an odd number of electrons per supercell. We argue that superstructure Mottness exists even in the absence of translation symmetry breaking by a superlattice, provided that the extended or intersite Coulomb interaction is strong. In the latter case, superstructure Mottness …
Using variational algorithm on an infinite Shastry-Sutherland (SS) lattice we show that the introduction of a static nonmagnetic impurity into a dimerized ground state leads to a formation of a small, localized spin polaron surrounding the impurity site. Due to a particular symmetry of the SS lattice, the polaron is extremely anisotropic with a short spatial extent. The presence of nonmagnetic impurities leads to a formation of pronounced in-gap peaks in the dynamical spin structure factor, which we attribute to the spin-doublet excitations of a single unpaired spin $S=1/2$ surrounded by tripl…
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 show that the strong coupling physics inherent to the insulating Mott state in 2D leads to a jump in the chemical potential upon doping and the emergence of a pseudogap in the single particle spectrum below a characteristic temperature. The pseudogap arises because any singly-occupied site not immediately neighbouring a hole experiences a maximum energy barrier for transport equal to $t^2/U$, where $t$ is the nearest-neighbour hopping integral and $U$ the on-site repulsion. The resultant pseudogap cannot vanish before each lattice site, on average, has at least one hole as a near neighbour.…
The chemical selectivity and great sensitivity of the Extended X-ray Absorption Spectroscopy technique allowed the determination, in the paramagnetic phase, of the structural distortions induced by doping in the spin-Peierls CuGeO$_3$ compound. The distorted environments were analyzed as a function of concentration, magnetic nature of impurity and the substitution site (Ni, Mn and Zn impurities on the Cu site, Si impurity on the Ge site). This has led to estimate the variation of the angles and pair distances, and hence to evaluate the magnetic coupling along the Cu chains in the vicinity of t…
We demonstrate that a Mott insulator lightly doped with holes is still an insulator at low temperature even without disorder. Hole localization obtains because the chemical potential lies in a pseudogap which has a vanishing density of states at zero temperature. The energy scale for the pseudogap is set by the nearest-neighbour singlet-triplet splitting. As this energy scale vanishes if transitions, virtual or otherwise, to the upper Hubbard band are not permitted, the fundamental length scale in the pseudogap regime is the average distance between doubly occupied sites. Consequently, the pse…
While both the hole and electron doped cuprates can exhibit $d_{x^2-y^2}$-wave superconductivity, the local distribution of the doped carriers is known to be significantly different with the doped holes going primarily on the O sites while the doped electrons go on the Cu sites. Here we report the results of density matrix renormalization calculations for a three-orbital model of a CuO$_2$ lattice. In addition to the asymmetric dependence of the intra-unit-cell occupation of the Cu and O for hole and electron doping, we find important differences in the longer range spin and charge correlation…
We present a numerical study of the doping dependence of the spectral function of the n-type cuprates. Using a variational cluster-perturbation theory approach based upon the self-energy-functional theory, the spectral function of the electron-doped two-dimensional Hubbard model is calculated. The model includes the next-nearest neighbor electronic hopping amplitude $t'$ and a fixed on-site interaction $U=8t$ at half filling and doping levels ranging from $x=0.077$ to $x=0.20$. Our results support the fact that a comprehensive description of the single-particle spectrum of electron-doped cupra…
High Tc superconductors show a rich variety of phases associated with their charge degrees of freedom. Valence charges can give rise to charge ordering or acoustic plasmons in these layered cuprate superconductors. While charge ordering has been observed for both hole- and electron-doped cuprates, acoustic plasmons have only been found in electron-doped materials. Here, we use resonant inelastic X-ray scattering (RIXS) to observe the presence of acoustic plasmons in two families of hole-doped cuprate superconductors [La2-xSrxCuO4 (LSCO) and Bi2Sr1.6La0.4CuO6+d (Bi2201)], crucially completing t…
The nuclear spin-lattice relaxation rate ($1/T_{1}$) has been measured in the single crystals of hole doped two-leg ladder compounds Sr$_{14-x}$Ca$_{x}$Cu$_{24}$O$_{41}$ and in the undoped parent material La$_6$Ca$_8$Cu$_{24}$O$_{41}$. Comparison of $1/T_{1}$ at the Cu and the two distinct oxygen sites revealed that the major spectral weight of low frequency spin fluctuations is located near $q \sim (π, π)$ for most of the temperature and doping ranges investigated. Remarkable difference in the temperature dependence of $1/T_1$ for the two oxygen sites in the heavily doped $x$=12 sample reveal…
We report preparation, x-ray diffraction, magnetic susceptibility chi(T) and heat capacity Cp(T) measurements on the undoped samples as also samples with Zn-doped (S = 0) at Cu site, Ni doped (S = 1) at Cu site, and Ca-doped (holes) at Bi site in the coupled two-leg spin ladder system BiCu2PO6. While, Zn shows complete solid solubility, Ni could be doped to about 20% and Ca to about 15%. Magnetization and heat capacity data in the undoped compound point towards the existence of frustration effects. In all the samples, the chi(T) at low temperature increases with doping content. The Zn-induced …
We use resonant inelastic x-ray scattering (RIXS) at the Ir L3 edge to study the effect of hole doping upon the Jeff=1/2 Mott-insulating state in Sr2IrO4, via Rh replacement of the Ir site. The spin-wave gap, associated with XY-type spin-exchange anisotropy, collapses with increasing Rh content, prior to the suppression of the Mott-insulating state and in contrast to electron doping via La substitution of the Sr site. At the same time, despite heavy damping, the d-d excitation spectra retain their overall amplitude and dispersion character. A careful study of the spin-wave spectrum reveals tha…
Magnetic ground states of highly doped two-leg Hubbard ladders with a particle bath
Advanced Science Research Center, Japan Atomic Energy Agency（日本原子能机构先进科学研究中心）；Department of Physics, Graduate School of Science, The University of Tokyo（东京大学理学研究科物理学系）；JSR-UTokyo Collaboration Hub, CURIE（JSR-东京大学合作中心，CURIE）
We suggest that when zinc is substituted for copper in the copper oxide planes of high $T_{c}$ superconductors, it does not necessarily have a valency of 2+. Rather, the valency of a zinc impurity should be determined by its surrounding medium. In order to study this hypothesis, we examine the effect of static impurities inducing diagonal disorder within a one band Hubbard model coupled to a localised state. We use this model to discuss the physics of zinc doping in the cuprates. Specifically, we discuss the formation of local moments near impurity sites and the modification of the transverse …
We show how a lightly doped Mott insulator has hugely enhanced electronic thermal transport at low temperature. It displays universal behavior independent of the interaction strength when the carriers can be treated as nondegenerate fermions and a nonuniversal "crossover" region where the Lorenz number grows to large values, while still maintaining a large thermoelectric figure-of-merit. The electron dynamics are described by the Falicov-Kimball model which is solved for arbitrary large on-site correlation with a dynamical mean-field theory algorithm on a Bethe lattice. We show how these resul…
It is widely believed that high-temperature superconductivity in the cuprates emerges from doped Mott insulators. The physics of the parent state seems deceivingly simple: The hopping of the electrons from site to site is prohibited because their on-site Coulomb repulsion U is larger than the kinetic energy gain t. When doping these materials by inserting a small percentage of extra carriers, the electrons become mobile but the strong correlations from the Mott state are thought to survive; inhomogeneous electronic order, a mysterious pseudogap and, eventually, superconductivity appear. How th…
By systematic chemical substitution of Pt and Ni in the newly-discovered superconductor Nb$_2$PdS$_5$ ($T_c\sim$6 K), we study the evolution of its superconducting properties with doping, focussing on the behavior of the upper critical field $H_{c2}$. In contrast to the previous results of Se doping on S sites, superconductivity is found to be rather robust against the Pt and Ni dopants on the one-dimensional Pd chains. Most strikingly, the reduced $H_{c2}$, i.e., the ratio of $H_{c2}/T_c$, is seen to be significantly enhanced by the heavier Pt doping but suppressed in the Ni-doped counterpart…
We observed slow relaxation of magnetoresistance in quantum well structures GaAs-AlGaAs with a selective doping of both wells and barrier regions which allowed partial filling of the upper Hubbard band. Such a behavior is explained as related to magnetic-field driven redistribution of the carriers between sites with different occupation numbers due to spin correlation on the doubly occupied centers. This redistribution, in its turn, leads to slow multi-particle relaxations in the Coulomb glass formed by the charged centers.
