Impurity-induced Magnetism Research Articles

Impurity-Induced Magnetization of Graphene.

We present a model of impurity-induced magnetization of graphene assuming that the main source of graphene magnetization is related to impurity states with a localized spin. The analysis of solutions of the Schrödinger equation for electrons near the Dirac point has been performed using the model of massless fermions. For a single impurity, the solution of Schrödinger’s equation is a linear combination of Bessel functions. We found resonance energy levels of the non-magnetic impurity. The magnetic moment of impurity with a localized spin was accounted for the calculation of graphene magnetization using the Green’s function formalism. The spatial distribution of induced magnetization for a single impurity is obtained. The energy of resonance states was also calculated as a function of interaction. This energy is depending on the impurity potential and the coupling constant of interaction.

Effects of biaxial strain on the impurity-induced magnetism in P-doped graphene and N-doped silicene: a first principles study

The effects of biaxial strain on the impurity-induced magnetism in P-doped graphene (P-graphene) and N-doped silicene (N-silicene) are studied by means of spin-polarized density functional calculations, using the supercell approach. The calculations were performed for three different supercell sizes 4 × 4, 5 × 5, and 6 × 6, in order to simulate three different dopant concentrations 3.1, 2.0 and 1.4%, respectively. For both systems, the calculated magnetic moment is 1.0 μB per impurity atom for the three studied concentrations. From the analysis of the electronic structure and the total energy as a function of the magnetization, we show that a Stoner-type model describing the electronic instability of the narrow impurity band accounts for the origin of sp-magnetism in P-graphene and N-silicene. Under biaxial strain the impurity band dispersion increases and the magnetic moment gradually decreases, with the consequent collapse of the magnetization at moderate strain values. Thus, we found that biaxial strain induces a magnetic quantum phase transition in P-graphene and N-silicene.

Symmetric single-impurity Kondo model on a tight-binding chain: Comparison of analytical and numerical ground-state approaches

We analyze the ground-state energy, local spin correlation, impurity spin polarization, impurity-induced magnetization, and corresponding zero-field susceptibilities of the symmetric single-impurity Kondo model on a tight-binding chain with bandwidth $W=2{\cal D}$ and coupling strength $J_{\rm K}$. We compare perturbative results and variational upper bounds from Yosida, Gutzwiller, and first-order Lanczos wave functions to the numerically exact data obtained from the Density-Matrix Renormalization Group (DMRG) and from the Numerical Renormalization Group (NRG) methods. The Gutzwiller variational approach becomes exact in the strong-coupling limit and reproduces the ground-state properties from DMRG and NRG for large couplings. We calculate the impurity spin polarization and its susceptibility in the presence of magnetic fields that are applied globally/locally to the impurity spin. The Yosida wave function provides qualitatively correct results in the weak-coupling limit. In DMRG, chains with about $10^3$ sites are large enough to describe the susceptibilities down to $J_{\rm K}/{\cal D}\approx 0.5$. For smaller Kondo couplings, only the NRG provides reliable results for a general host-electron density of states $\rho_0(\epsilon)$. To compare with results from Bethe Ansatz, we study the impurity-induced magnetization and zero-field susceptibility. For small Kondo couplings, the zero-field susceptibilities at zero temperature approach $\chi_0(J_{\rm K}\ll {\cal D})/(g\mu_{\rm B})^2\approx \exp[1/(\rho_0(0)J_{\rm K})]/(2C{\cal D}\sqrt{\pi e \rho_0(0)J_{\rm K}})$, where $\ln(C)$ is the regularized first inverse moment of the density of states. Using NRG, we determine the universal sub-leading corrections up to second order in $\rho_0(0)J_{\rm K}$.

Theoretical study of impurity-induced magnetism in FeSe

Experimental evidence suggests that FeSe is close to a magnetic instability, and recent scanning tunneling microscopy (STM) measurements on FeSe multilayer films have revealed stripe order locally pinned near defect sites. Motivated by these findings, we perform a theoretical study of locally induced magnetic order near nonmagnetic impurities in a model relevant for FeSe. We find that relatively weak repulsive impurities indeed are capable of generating short-range magnetism, and explain the driving mechanism for the local order by resonant eg-orbital states. In addition, we investigate the importance of orbital-selective self-energy effects relevant for Hund's metals, and show how the structure of the induced magnetization cloud gets modified by orbital selectivity. Finally, we make concrete connection to STM measurements of iron-based superconductors by symmetry arguments of the induced magnetic order, and the basic properties of the Fe Wannier functions relevant for tunneling spectroscopy.

μSR Studies on Magnetism in High-Tc Cuprates

Since the discovery of high-Tc superconductivity in cuprates, muon spin relaxation (μSR) measurements have greatly contributed to the understanding of high-Tc superconductivity. In this paper, μSR studies on the magnetism in high-Tc cuprates obtained these past three decades are reviewed. Antiferromagnetic long-range order, 1/8 anomaly, stripes of Cu spins and holes, impurity-induced magnetism, magnetic-field-induced magnetism, pseudogap, ferromagnetism in the heavily overdoped regime, and undoped superconductivity in T′-type cuprates are discussed. Moreover, the fundamentals of μSR measurements for the study of magnetism are described for μSR beginners.

Impurity-induced magnetization in a three-dimensional antiferromagnet at the quantum critical point

We consider a single impurity with spin $S$ embedded in a three-dimensional antiferromagnetic system which is close to the quantum critical point (QCP), separating magnetically ordered and disordered phases. Approaching the QCP from the disordered phase we study the spatial distribution of spin density and staggered magnetization induced by the impurity. Using two methods (self-consistent Born approximation and renormalization group) we found a power law decay of the spin density $\propto 1/r^3$, and of the staggered magnetization $\propto 1/r$ with relevant logarithmic corrections. We demonstrate that the local spin at the impurity site $r=0$ approaches to zero at the QCP. We show that in the semiclassical limit of large $S$ the problem is equivalent to the exactly solvable independent boson model. Our results demonstrate existence of spin-charge separation in the three dimensional systems in the vicinity of the QCP.

Local Magnetization Nucleated by Non-magnetic Impurities in Fe-based Superconductors

We study impurity-induced magnetic order within a five-band Hubbard model relevant to the normal paramagnetic phase of iron-based superconductors. The existence of the local magnetic order is explained in terms of an impurity-enhancement of states near the Fermi level, and we map out the resulting phase diagram of the existence of magnetization as a function of impurity strength and Coulomb correlations. In particular, the presence of impurity-induced magnetism in only a certain range of potential scattering strengths can be understood from the specific behavior of the impurity resonant state.

Interstitial impurity-induced magnetism in α-PbO surface

We have investigated the effects of 3d transition metal (TM) and non-magnetic interstitial impurities in α-PbO (0 0 1) surface using ab-initio calculations. The calculated impurity-induced magnetic moments are 2.25 μB, 3.11 μB and 0.94 μB for Fe, Mn and Pb interstitials respectively. In the bonding process, TM's lower energy lying states form overlaps with nearest neighbour oxygen atoms' pz states, with other non-bonding spin split d states situated near or at the Fermi level. These spin split orbitals introduce spin polarised p impurity states of oxygen atoms near the surface.

Impurity-induced subgap bound states in alkali-doped iron chalcogenide superconductors

Measurements of the local density of states near impurities can be useful for identifying the superconducting gap structure in alkali doped iron chalcogenide superconductors K_xFe_{2-y}Se_2. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest neighbor d-wave and next-nearest neighbor s-wave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that sub-gap bound states exist only for d-wave superconductors with the positions of these bound states depending rather sensitively on the electron doping level. Further, for such disorder Coulomb interactions can lead to local impurity-induced magnetism in the case of d-wave superconductivity. For magnetic impurities, both s-wave and d-wave superconducting states support sub-gap bound states. The above results can be explained by a simple analytic model that provides a semi-quantitative understanding of the variation of the impurity bound states energies as a function of impurity potential and chemical doping level.

Impurity and magnetic field effects on the stripes in cuprates

Impurity and magnetic field effects on the stripes are reviewed not only in the La-based cuprates but also in the other high-Tc cuprates. It has turned out that the so-called 1/8 anomaly takes place not only in the La-based cuprates but also in the hole-doped high-Tc cuprates universally, when adequate pinning centers are introduced. Impurity-induced magnetism tends to emerge at low temperatures in a wide range of hole concentration in LSCO, Bi2201 and YBCO where superconductivity appears. Accordingly, it is possible that the dynamically fluctuating stripes exist universally in the hole-doped high-Tc cuprates and play an important role in the appearance of the high-Tc superconductivity. For the hole-doped high-Tc cuprates, it has turned out that magnetic field effects on the stripes are small in samples where the stripes are well stabilized statically in zero field, while field-induced magnetism is observed under magnetic fields parallel to the c-axis in underdoped samples where magnetism is not well developed in zero field. The field-induced magnetism has been observed in some optimally doped samples also, but it has never been observed in overdoped samples. Field-induced CDW has been observed in slightly overdoped BSCCO. These results are understood fundamentally in terms of the theoretical phase diagram as a function of magnetic field, based on the Ginzburg–Landau theory with competing antiferromagnetic and superconducting order parameters. It is also possible to understand that the field-induced magnetism and CDW are due to pinning of the dynamically fluctuating stripes by vortex cores as in the case of the pinning by impurities. In the electron-doped high-Tc cuprates, on the other hand, impurity-induced magnetism has never been observed. The nature of the field-induced magnetism observed in the electron-doped cuprates is different from that in the hole-doped ones.

Impurity-Induced Magnetization of Layered Semiconductor LaCuSeO as Predicted from First-Principles Calculations

Using the ab initio FLAPW-GGA method, we systematically examined the effects of various doping types on the magnetic and electronic properties of the layered semiconductor LaCuSeO. Magnetization of LaCuSeO induced by 3d impurities (M=Mn, Fe, and Co) in the Cu sublattice was found as a result of spontaneous spin polarization of M 3d↑↓ impurity bands. Partially filled M 3d↓ bands lie in the gap of LaCuSeO and are crossed by EF, whereas M 3d↑ bands are admixed to the top of the valence band resulting in a magnetic half-metallic behavior of these doped systems. The other possible types of doping (i.e., introduction of arsenic atoms into Se sublattice as well as the introduction of N or F atoms into an oxygen sublattice) remain the semiconductor LaCuSeO non-magnetic, but result in the transition into metallic-like state.

Defects in correlated metals and superconductors

In materials with strong local Coulomb interactions, simple defects such as atomic substitutions strongly affect both macroscopic and local properties of the system. A nonmagnetic impurity, for instance, is seen to induce magnetism nearby. Even without disorder, models of such correlated systems are generally not soluble in 2 or 3 dimensions, and so few exact results are known for the properties of such impurities. Nevertheless, some simple physical ideas have emerged from experiments and approximate theories. Here, we first review what we can learn about this problem from 1D antiferromagnetically correlated systems. We then discuss experiments on the high Tc cuprate normal state which probe the effect of impurities on local charge and spin degrees of freedom, and compare with theories of single impurities in correlated hosts, as well as phenomenological effective Kondo descriptions. Subsequently, we review theories of impurities in d-wave superconductors including residual quasiparticle interactions, and compare with experiments in the superconducting state. We argue that existing data exhibit a remarkable similarity to impurity-induced magnetism in the 1D case, implying the importance of electronic correlations for the understanding of these phenomena, and suggesting that impurities may provide excellent probes of the still poorly understood ground state of the cuprates.

Magnetic interaction between impurity and impurity-liberated spins in the doped Haldane chain compoundsPbNi2−xAxV2O8(A=Mg,Co)

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 observed severe broadening of the ESR lines in the magnetically doped compounds with respect to nonmagnetic doping is attributed to a rapid spin-lattice relaxation of the Co${}^{2+}$ ions, which results in a bottleneck-type of temperature dependence of the induced linewidth. The exchange parameters obtained from the ESR analysis offer a satisfactory explanation of the observed low-temperature magnetization in doped samples.

Correlation length in cuprate superconductors deduced from impurity-induced magnetization

We report a new multinuclei based nuclear magnetic resonance method which allows us to image the staggered polarization induced by nonmagnetic $\mathrm{Li}$ impurities in underdoped $({\mathrm{O}}_{6.6})$ and slightly overdoped $({\mathrm{O}}_{7})$ ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ above ${T}_{C}$. The spatial extension of the polarization ${\ensuremath{\xi}}_{\mathrm{imp}}$ approximately follows a Curie law, increasing up to six lattice constants at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ at ${\mathrm{O}}_{6.6}$ in the pseudogap regime. Near optimal doping, the staggered magnetization has the same shape, with ${\ensuremath{\xi}}_{\mathrm{imp}}$ reduced by a factor 2. ${\ensuremath{\xi}}_{\mathrm{imp}}$ is argued to reveal the intrinsic magnetic correlation length of the pure system. It is found to display a smooth evolution through the pseudogap regime.

Universal behavior of spinless defects in the Haldane chain Y2Ba(Ni,Zn)O5: a study via bulk (squid) and local (NMR) probes

Abstract Spinless-impurity induced magnetic effects in Y 2 Ba(Ni,Zn)O 5 have been probed via susceptibility and 89 Y NMR. 89 Y NMR spectra of these samples contain multiple peaks, as a consequence of impurity-induced excitations. Here, the T -dependence of the impurity-induced magnetization is consistent with a net “effective” spin S ≈0.4 on either side of the Zn-impurity in the one-dimensional chain. Further, 89 Y NMR experiments with (Y 1− y Ca y ) 2 BaNi 1− x Zn x O 5 , indicate a non-interacting behavior of the hole (introduced due to Ca doping) and S =0 impurity.