We report the results of 151Eu Moessbauer effect and magnetization measurements in the Eu-doped Ca3Co2O6 and Ca3CoRhO6, which are of great current interest in the fields of spin-chain magnetism and geometrical frustration. We find that there is a pronounced increase in the line-width of the Moessbauer spectra below a certain characteristic temperature which is well-above the one at which three-diensional ordering features set in. This unusual broadening of the spectra indicates the existence of a characteristic temperature in these 'exotic' magnetic systems, attributable to the onset of incipient one-dimensional magnetic order. This is inferred from an intriguing correlation of this characteristic temperature with the paramagnetic Curie temperature (a measure of intrachain coupling strength in these cases).
The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and 151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0 (parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found that spin density wave (SDW) is suppressed by Co-substitution, however it survives in the region of superconductivity, but iron spectra exhibit some non-magnetic component in the superconducting region. Europium orders anti-ferromagnetically regardless of the Co concentration with the spin re-orientation from the a-axis in the parent compound toward c-axis with the increasing…
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 …
Combining LSDA+$U$ and an analysis of superexchange interactions beyond DFT, we describe the magnetic ground states in rutile and anatase Cr-doped TiO$_2$. In parallel, we correct our LSDA+$U$ ground state through GW corrections ($GW$@LSDA+$U$) that reproduce the position of impurity states and the band gaps in satisfying agreement with experiments. Because of the different topological coordinations of Cr-Cr bonds in the ground states of rutile and anatase, superexchange interactions induce either ferromagnetic or antiferromagnetic couplings of Cr ions. In Cr-doped anatase, this interaction le…
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…
Neutron measurements on Ca{1-x}La{x}MnO3 (0.00 <= x <= 0.20) reveal the development of a liquid-like spatial distribution of magnetic droplets of average size ~10 Angstroms, the concentration of which is proportional to x (one cluster per ~60 doped electrons). In addition, a long-range ordered ferromagnetic component is observed for ~0.05 < x < ~0.14. This component is perpendicularly coupled to the simple G-type antiferromagnetic (G-AFM) structure of the undoped compound, which is a signature of a G-AFM + FM spin-canted state. The possible relationship between cluster formation and the stabil…
In order to investigate whether magnetism and superconductivity coexist in Co-doped SrFe$_2$As$_2$, we have prepared single crystals of SrFe$_{2-x}$Co$_x$As$_2$, $x$ = 0 and 0.4, and characterized them via X-ray diffraction, electrical resistivity in zero and applied field up to 9 T as well as at ambient and applied pressure up to 1.6 GPa, and magnetic susceptibility. At $x$ = 0.4, there is both magnetic and resistive evidence for a spin density wave transition at 120 K, while $T_c$ = 19.5 K - indicating coexistent magnetism and superconductivity. A discussion of how these results compare with…
Using the recently developed method we calculate the crystal field parameters in yttrium and lutetium aluminum garnets doped with seven trivalent Kramers rare-earth ions. We then insert calculated parameters into the atomic-like Hamiltonian taking into account the electron-electron, spin-orbit and Zeeman interactions and determine the multiplet splitting by the crystal field as well as magnetic $\hat{g}$ tensors. We compare calculated results with available experimental data.
Considering certain interesting features in the previously reported 166Er Moessbauer effect and neutron diffraction data on the polycrystalline form of ErPd2Si2 crystallizing in ThCr2Si2-type tetragonal structure, we have carried out magnetic measurements (1.8 to 300 K) on the single crystalline form of this compound. We observe significant anisotropy in the absolute values of magnetization (indicating that the easy axis is c-axis) as well as in the features due to magnetic ordering in the plot of magnetic susceptibility (chi) versus temperature (T) at low temperatures. The chi(T) data reveal …
We studied the nano-scale structure of the short-range incommensurate magnetic order in La1.5Sr0.5CoO4 by elastic neutron scattering. We find that magnetic diffuse scattering is isotropic in the a-b plane, in contrast with the naive expectation based on the popular stripe model. Indeed, charge segregation into lines favoring certain lattice direction(s) would facilitate linear stacking faults in an otherwise robust antiferromagnetism of un-doped material, leading to anisotropic disorder, with a characteristic symmetry pattern present in the neutron scattering data.
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…
The hole-doped kagome lattice of Cu2+ ions in LaCuO2.66 was investigated by nuclear quadrupole resonance (NQR), electron spin resonance (ESR), electrical resistivity, bulk magnetization and specific heat measurements. For temperatures above ~180 K, the spin and charge properties show an activated behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be directly detected here, the observation of coexi…
We have studied the electronic structure of Zn$_{0.9}$Fe$_{0.1}$O nano-particles, which have been reported to show ferromagnetism at room temperature, by x-ray photoemission spectroscopy (XPS), resonant photoemission spectroscopy (RPES), x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). From the experimental and cluster-model calculation results, we find that Fe atoms are predominantly in the Fe$^{3+}$ ionic state with mixture of a small amount of Fe$^{2+}$ and that Fe$^{3+}$ ions are dominant in the surface region of the nano-particles. It is shown that the roo…
We report a series of magnetic and transport measurements on high-quality single crystal samples of colossal magnetoresistive manganites, La_{0.7} Ca_{0.3} Mn O_3 and Pr_{0.7} Sr_{0.3} Mn O_3. 1 % Fe doping allows a Moessbauer spectroscopy study, which shows (i) unusual line broadening within the ferromagnetic phase and (ii) a coexistence of ferro- and paramagnetic contributions in a region, T_1<T<T_2, around the Curie point T_C. In the case of Pr_{0.7} Sr_{0.3} Mn O_3, the resistivity peak occurs at a considerably higher temperature, T_{MI}>T_2. This shows that phase separation into metallic …
We have used a combination of resonant magnetic x-ray scattering (RMXS) and x-ray absorption spectroscopy (XAS) to investigate the properties of the doped spin-orbital Mott insulator Sr2Ir(1-x)Rh(x)O4 (0.07 < x < 0.70). We show that Sr2Ir(1-x)Rh(x)O4 represents a unique model system for the study of dilute magnetism in the presence of strong spin-orbit coupling, and provide evidence of a doping-induced change in magnetic structure and a suppression of magnetic order at x_c ~ 0.17. We demonstrate that Rh-doping introduces Rh3+/Ir5+ ions which effectively hole-dope this material. We propose that…
A review of the magnetism in the parent compounds of the iron-based superconductors is given based on the transmission Moessbauer spectroscopy of 57Fe and 151Eu. It was found that the 3d magnetism is of the itinerant character with varying admixture of the spin-polarized covalent bonds. For the 122 compounds a longitudinal spin density wave (SDW) develops. In the case of the EuFe2As2 a divalent europium orders antiferromagnetically at much lower temperature as compared to the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe(1+x)Te p…
We report an x-ray emission spectroscopy (XES) study of the local fluctuating magnetic moment ($μ_{bare}$) in $\mathrm{NaFe_{1-x}Co_{x}As}$ and $\mathrm{NaFe_{1-x}Cu_{x}As}$. In NaFeAs, the reduced height of the As ions induces a local magnetic moment higher than $\mathrm{Ba_2As_2}$, despite lower T$_N$ and ordered magnetic moment. As NaFeAs is doped with Co $μ_{bare}$ is slightly reduced, whereas Cu doping leaves it unaffected, indicating a different doping mechanism: based on electron counting for Co whereas impurity scattering dominates in the case of Cu. Finally, we observe an increase of …
Here, we report an investigation of structural, magnetic and electronic properties in Y-doped layered iridate (Sr$_{1-x}$Y$_x$)$_2$IrO$_4$ ($x$ $\leq$ 0.1). The parent Sr$_2$IrO$_4$ is a well-studied spin-orbit coupling (SOC) induced insulator with an antiferromagnetic ground state. The Y-doping here equivalently acts for electron doping without altering the vital parameters such as, SOC and electron correlation. Experimental results show a minute change in structural parameters and an equivalent charge conversion from Ir$^{4+}$ to Ir$^{3+}$. Unlike similarly other electron-doped system, the l…
By bosonizing the electronic t-J model exactly on any two-dimensional (2D) lattices, and integrating out the gauge fluctuations combined to slave particles beyond mean fields, we get a theory in terms of physical Cooper pair and spin condensates. In the sense of mutual Berry phase they turns out to be dual to each other. The mutual-duality is the missing key in the resonant-valance-bond idea\cite{rvb} to work as a paradigm of doped 2D Mott insulators. We argue that essential aspects of high-$T_c$ phenomenology find natural solutions in the theory. We also provide interesting predictions for sy…
A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The combination of neutron diffraction and angle-resolved photoemission spectroscopy (ARPES) measurements show that the tuning of a flat band with $d_{x^2-y^2}$ orbital character drives the helimagnetism a…
Existing data on Cu-nuclear spin relaxation reveal two independent relaxation processes: the one that is temperature independent we link to incommensurate peaks seen by neutrons, while the ''universal'' temperature dependent contribution coincides with 1/{63}^T_{1}(T) for two-chain YBCO 124. We argue that this new result substitutes for a ''pseudogap'' regime in a broad class of high-T_c cuprates and stems from the 1st order phase transition that starts well above the superconductivity T_c but becomes frustrated because of broken electroneutrality in the CuO_2 plane.
Magnetization measurements of Sr{14-x}Ca_xCu_{24}O_{41} with 0 <= x <=12 in magnetic fields up to 16 T show that the low-temperature magnetic response of the CuO_2 spin chains changes strongly upon Ca doping. For x=0 quantum statistical simulations yield that the temperature and field dependence of the magnetization can be well described by an effective Heisenberg model in which the ground state configuration is composed of spin dimers, trimers, and monomers. For x>0 a constant contribution to the low-temperature magnetic susceptibility is observed which cannot be explained in terms of simple …
The $S=1/2$ antiferromagnetic Heisenberg spin chain compound Sr$_2$CuO$_3$ doped with $1 \%$ and $2 \%$ of Ni impurities has been studied by means of $^{63}$Cu nuclear magnetic resonance. A strong decrease of the spin-lattice relaxation rate $T_1^{-1}$ at low temperatures points toward a spin gap, while a stretching exponent $λ< 1$ and a frequency dependence of $T_1^{-1}$ indicate that this spin gap varies spatially and should rather be characterized as a spin pseudogap. The magnitude of the spin pseudogap scales with doping level. Our results therefore evidence the finite-size character of th…
The ESR signal of Gd spin probes (0.5 at %) as well as the static normal state susceptibility of Eu (J(Eu^{3+})=0) doped La_{2-x-y}Sr_xEu_yCuO_4 reveal pronounced changes of the Cu magnetism at the structural transition from the orthorhombic to the low temperature tetragonal phase for all non-superconducting compositions. Both a jumplike decrease of χas well as the ESR data show an increase of the in-plane magnetic correlation length in the LTT phase. From the Gd^{3+} ESR linewidth we find that for specific Eu and Sr concentrations in the LTT phase the correlation length increases up to more t…
The compound BaFe2Se3 (Pnma) has been synthesized in the form of single crystals with the average composition Ba0.992Fe1.998Se3. The Moessbauer spectroscopy used for investigation of the valence states of Fe in this compound at temperature ranging from 4.2 K till room temperature revealed the occurrence of mixed-valence state for iron. The spectrum is characterized by sharply defined electric quadrupole doublet above magnetic ordering at about 250 K. For the magnetically ordered state one sees four iron sites at least and each of them is described by separate axially symmetric electric field g…
We demonstrate that room-temperature ferromagnetism in epitaxial Co-doped TiO2 anatase is driven by electron-mediated exchange interaction, and not by metallic Co clusters. Co(II) substitutes for Ti(VI) in the lattice and produces oxygen vacancies that do not contribute carriers. Free electrons originate with oxygen vacancies resulting from an oxygen deficiency during growth.
We study the dynamics of a single hole in Li and Sr doped La$_{2}$CuO$_4$ and its extension to a finite hole concentration. We compare the physics of La$_{2-x}$Sr$_x$CuO$_4$ and La$_{2}$Cu$_{1-x}$Li$_x$O$_4$ and explain why these systems are remarkably different. We demonstrate that holes in La$_{2}$Cu$_{1-x}$Li$_x$O$_4$ are always localized and that there is a critical concentration, $x_c\approx 0.03$, above which the holes break the global antiferromagnetic state into an array of weakly coupled antiferromagnetic clusters (antiferromagnetic shards). We show that the spin-shard state provides …
The understanding of the interplay between different orders in a solid is a key challenge in highly correlated electronic systems. In real systems this is even more difficult since disorder can have a strong influence on the subtle balance between these orders and thus can obscure the interpretation of the observed physical properties. Here we present a study on delta-doped La2CuO4 superlattices. By means of molecular beam epitaxy whole LaO-layers were periodically replaced through SrO-layers providing a charge reservoir, yet reducing the level of disorder typically present in doped cuprates t…
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…
The fine control of magnetism and electronic structure is crucial since the interplay between magnetism and band topology can lead to various novel magnetic topological states including axion insulators, magnetic Weyl semimetals and Chern insulators etc. Through crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that with Sb-doping, the newly-discovered intrinsic antiferromagnetic topological insulator MnBi4Te7 evolves from antiferro-magnetic to ferromagnetic and then ferrimagnetic. We attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which re…
A well-known feature of CaFe$_{2}$As$_{2}$-based superconductors is the pressure-induced collapsed tetragonal phase that is commonly ascribed to the formation of an interlayer As-As bond. Using detailed X-ray scattering and spectroscopy, we find that Rh-doped Ca$_{0.8}$Sr$_{0.2}$Fe$_{2}$As$_{2}$ does not undergo a first-order phase transition and that local Fe moments persist despite the formation of interlayer As-As bonds. Our density functional theory calculations reveal that the Fe-As bond geometry is critical for stabilizing magnetism and that the pressure-induced drop in the $c$ lattice p…
Moessbauer spectroscopy measurements were performed for the temperature range between 4.2 K and 300 K in a transmission geometry applying 14.41-keV resonant line in 57Fe for PrFeAsO the latter being a parent compound of the iron-based superconductors belonging to the '1111' family. It was found that an itinerant 3d magnetic order develops at about 165 K and it is accompanied by an orthorhombic distortion of the chemical unit cell. A complete longitudinal 3d incommensurate spin density wave (SDW) order develops at about 140 K. Transferred hyperfine magnetic field generated by the praseodymium m…
A Kondo-lattice theory is applied to the crossover between local-moment magnetism for almost half fillings of electrons and itinerant-electron magnetism away from the half filling. In clean systems with no disorder, the bandwidth W^* of quasiparticles is non-zero and of the order of |J| at T=0K even in the limit of the half filling, with J the superexchange interaction constant between nearest neighbors. The so called Gutzwiller's term also contributes to W^* away from the half filling; it is approximately proportional to doping concentrations measured from the half filling. Magnetism is enhan…
Strong spin-lattice coupling and prominent frustration effects observed in the 50$\%$ Fe-doped frustrated hexagonal ($h$)LuMnO$_3$ are reported. A Néel transition at $T_{\mathrm N} \approx$ 112~K and a possible spin re-orientation transition at $T_{\mathrm {SR}} \approx$ 55~K are observed in the magnetization data. From neutron powder diffraction data, the nuclear structure at and below 300~K was refined in polar $P6_3cm$ space group. While the magnetic structure of LuMnO$_3$ belongs to the $Γ_4$ ($P6'_3c'm$) representation, that of LuFe$_{0.5}$Mn$_{0.5}$O$_3$ belongs to $Γ_1$ ($P6_3cm$) which…
The magnetism of the second antiferromagnetic phase (AF2) arising in the iron-based LaFeAsO$_{1-x}$H$_{x}$ superconductor for $x\gtrsim0.4$ was investigated by muon spin rotation measurements under hydrostatic pressure up to 2.6 GPa. The Néel temperature ($T_{\rm N}$) obtained for a sample with $x=0.51$ exhibits considerably greater sensitivity to pressure than that in the pristine antiferromagnetic phase (AF1, $x\lesssim0.06$). Moreover, while the AF1 phase is always accompanied by the structural transition (from tetragonal to orthorhombic) at a temperature ($T_{\rm s}$) which is slightly hig…
Unravelling the nature of doping-induced transition between a Mott insulator and a weakly correlated metal is crucial to understanding novel emergent phases in strongly correlated materials. For this purpose, we study the evolution of spectral properties upon doping Mott insulating states, by utilizing the cluster perturbation theory on the Hubbard and t-J-like models. Specifically, a quasi-free dispersion crossing the Fermi level develops with small doping, and it eventually evolves into the most dominant feature at high doping levels. Although this dispersion is related to the free electron …
$^{121,123}Sb$ nuclear quadrupole resonance (NQR) was applied to $Fe(Sb_{1-x}Te_x)_2$ in the low doping regime (\emph{x = 0, 0.01} and \emph{0.05}) as a microscopic zero field probe to study the evolution of \emph{3d} magnetism and the emergence of metallic behavior. Whereas the NQR spectra itself reflects the degree of local disorder via the width of the individual NQR lines, the spin lattice relaxation rate (SLRR) $1/T_1(T)$ probes the fluctuations at the $Sb$ - site. The fluctuations originate either from conduction electrons or from magnetic moments. In contrast to the semi metal $FeSb_2$ …
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 ground--state magnetic phase diagram is investigated for the two-- and three--dimensional $t$--$t'$ Hubbard model. We take into account commensurate ferro--, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often missed in previous investigations. We trace the influence of correlation effects on the stability of both spiral and collinear magnetic order by comparing the results of employing both the generalized non-correlated mean--field (Hartree--Fock) approximation and generalized slave boson app…
The recent observation of superconductivity in infinite-layer nickelates has brought intense debate on the established knowledge of unconventional superconductivity based on the cuprates. Despite many similarities, the nickelates differ from the cuprates in many characteristics, the most notable one among which is the magnetism. Instead of a canonical antiferromagnetic Mott insulator as the undoped cuprates, from which the superconductivity is generally believed to arise upon doping, the undoped nickelates show no sign of magnetic ordering in experiments. Through a combined density functional …
The interplay between spin-orbit coupling (SOC) and electron correlation ($U$) is considered for many exotic phenomena in iridium oxides. We have investigated the evolution of structural, magnetic and electronic properties in pyrochlore iridate Y$_2$Ir$_{2-x}$Ru$_{x}$O$_7$ where the substitution of Ru has been aimed to tune this interplay. The Ru substitution does not introduce any structural phase transition, however, we do observe an evolution of lattice parameters with the doping level $x$. X-ray photoemission spectroscopy (XPS) study indicates Ru adopts charge state of Ru$^{4+}$ and replac…
Room temperature ferromagnetism was observed in n-type Fe-doped In2O3 thin films deposited on c-cut sapphire substrates by pulsed laser deposition. Structure, magnetism, composition, and transport studies indicated that Fe occupied the In sites of the In2O3 lattice rather than formed any metallic Fe or other magnetic impurity phases. Magnetic moments of films were proved to be intrinsic and showed to have a strong dependence on the carrier densities which depended on the Fe concentration and its valance state as well as oxygen pressure.
Electronic and magnetic properties of DNA structures doped by simple and transition d- and f-metal ions (Gd, La, Cu, Zn, Au) are reviewed. Both one- and two dimensional systems are considered. A particular attention is paid to gadolinium and copper doped DNA systems, their unusual magnetism being treated. The problem of classical and quantum transport (including transfer of genetic information during replication and transcription) and electron localization in biological systems is discussed.
Diluted magnetic semiconductors possessing intrinsic static magnetism at high temperatures represent a promising class of multifunctional materials with high application potential in spintronics and magneto-optics. In the hexagonal Fe-doped diluted magnetic oxide, 6H-BaTiO$_{3-δ}$, room-temperature ferromagnetism has been previously reported. Ferromagnetism is broadly accepted as an intrinsic property of this material, despite its unusual dependence on doping concentration and processing conditions. However, the here reported combination of bulk magnetization and complementary in-depth local-p…
Correlation between magnetism and the Verwey transition in magnetite
AGH University of Krakow（克拉科夫AGH科技大学）；Institute of Solid State Physics, TU Wien（维也纳工业大学固体物理研究所）；Mössbauer Spectroscopy Laboratory, Institute of Technology, University of the National Education Commission（国家教育委员会理工学院穆斯堡尔谱学实验室）；The Henryk Niewodniczanski Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院亨利克·涅沃德尼察斯基核物理研究所）；Purdue University（普渡大学）；Univ. Grenoble Alpes（格勒诺布尔阿尔卑斯大学）；Institut Néel, CNRS & Univ. Grenoble Alpes（内尔研究所，法国国家科学研究中心与格勒诺布尔阿尔卑斯大学）；University of Zagreb（萨格勒布大学）
Emergent Coherence at the Edge of Magnetism: Low-Doped La2-xSrxCuO4+delta Revisited
B. Verkin Institute for Low Temperature Physics and Engineering of the Nat. Acad. of Sciences of Ukraine（乌克兰国家科学院低溫物理與工程B·維爾金研究所）；Smart Materials Group, Mads Clausen Institute, University of Southern Denmark, Sønderborg（南丹麦大学马茨·克拉斯恩学院智能材料组）；National Technical University «Kharkiv Polytechnic Institute», Kharkiv, Ukraine（乌克兰哈尔科夫国立技术大学（哈尔科夫理工学院））
We propose the realization of linear crystals of cold ions which contain different atomic species for investigating quantum phase transitions and frustration effects in spin system beyond the commonly discussed case of $s=1/2$. Mutual spin-spin interactions between ions can be tailored via the Zeeman effect by applying oscillating magnetic fields with strong gradients. Further, collective vibrational modes in the mixed ion crystal can be used to enhance and to vary the strength of spin-spin interactions and even to switch those forces from a ferro- to an antiferromagnetic character. We conside…
Single-crystal sample of chromium doped with ca.0.2 at.%119Sn was studied by means of a transmission Mössbauer spectroscopy in the temperature range of 310-315 K. An anomaly in the temperature behavior of the center shift was found at ca. 313 K, a temperature that coincides well with the Néel temperature of chromium. The anomaly gives evidence that the vibrations of atoms in the studied system are affected by the magnetism.
The relationship between magnetism and superconductivity has been one of the most discussed topics in iron-based superconductors. Using the first-principles calculations, we have studied the electronic structure of 1144-type iron-based superconductor EuRbFe$_4$As$_4$. We find the crystal field splitting of EuRbFe$_4$As$_4$ is unique, such that the $d_{z^2}$ orbitals are closer to the Fermi level $ε_F$ than the $d_{xy}$ orbitals. The RKKY interaction strength is estimated to be approximately 0.12 meV in prestine EuRbFe$_4$As$_4$. Upon Ni-doping on the Fe site, the RKKY interaction strength is b…
A theory of Kondo lattices is applied to the crossover between local-moment magnetism and itinerant-electron magnetism in the t-J model on a quasi-two dimensional lattice. The Kondo temperature T_K is defined as a characteristic temperature or energy scale of local quantum spin fluctuations. Magnetism with T_N >> T_K, where T_N is the N\eel temperature, is characterized as local-moment one, while magnetism with T_N << T_K is characterized as itinerant-electron one. The Kondo temperature, which also gives a measure of the strength of the quenching of magnetic moments, is renormalized by the Foc…
Magnetism in La$_{1-x}$Sr$_x$CoO$_3$ as a function of doping is investigated with X-ray absorption spectroscopy and X-ray magnetic circular dichroism at the O K edge, and corresponding first principles electronic structure calculations. For small x, the spectra are consistent with the formation of ferromagnetic clusters occurring within a nonmagnetic insulating matrix. Sr-induced, magnetic O-hole states form just above E$_F$ and grow with increasing Sr doping. Density functional calculations for x = 0 yield a nonmagnetic ground state with the observed rhombohedral distortion and indicates that…
Epitaxial thin films of undoped LaCoO$_{3}$\@, of electron-doped La$_{0.7}$Ce$_{0.3}$CoO$_{3}$\@, and of hole-doped La$_{0.7}$Sr$_{0.3}$CoO$_{3}$\@ exhibit ferromagnetic order with a transition temperature $T_{\rm{C}}$ $\approx$ 85 K, 22 K, and 240 K, respectively. The spin-state structure for these compounds was studied by soft x-ray magnetic circular dichroism at the Co $L_{2,3}$ and O $K$ edges. It turns out that a higher spin state of the Co$^{3+}$ ions is responsible for the magnetism in LaCoO$_{3}$\@ while for La$_{0.7}$Ce$_{0.3}$CoO$_{3}$ the Co$^{3+}$ ions are in a low-spin state and t…
We present $^{75}$As Nuclear Magnetic and Quadrupole Resonance results (NMR, NQR) on a new set of LaFeAsO$_{1-x}$F$_x$ polycrystalline samples. Improved synthesis conditions led to more homogenized samples with better control of the fluorine content. The structural$\equiv$nematic, magnetic, and superconducting transition temperatures have been determined by NMR spin-lattice relaxation rate and AC susceptibility measurements. The so-determined phase diagram deviates from the published one especially for low F-doping concentrations. However, if the doping level is determined from the NQR spectra…
We report Eu-local-spin magnetism and Ni-doping-induced superconductivity (SC) in a 112-type ferroarsenide system Eu(Fe$_{1-x}$Ni$_{x}$)As$_2$. The non-doped EuFeAs$_2$ exhibits two primary magnetic transitions at $\sim$100 and $\sim$ 40 K, probably associated with a spin-density-wave (SDW) transition and an antiferromagnetic ordering in the Fe and Eu sublattices, respectively. Two additional successive transitions possibly related to Eu-spin modulations appear at 15.5 and 6.5 K. For the Ni-doped sample with $x$ = 0.04, the SDW transition disappears, and SC emerges at $T_\mathrm{c}$ = 17.5 K. …
A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution $^{…
