Effect Of Magnetic Impurities Research Articles

Magnetic impurity resonance states and symmetry of the superconducting order parameter in iron-based superconductors

We investigate the effect of magnetic impurities on the local quasiparticle density of states (LDOS) in iron-based superconductors. Employing the two-orbital model where 3$d$ electron and hole conduction bands are hybridizing with the localized $f$-orbital of the impurity spin, we investigate how various symmetries of the superconducting gap and its nodal structure influence the quasiparticle excitations and impurity bound states. We show that the bound states behave qualitatively different for each symmetry. Most importantly we find that the impurity-induced bound states can be used to identify the nodal structure of the extended s-wave symmetry ($S^{\pm}$) that is actively discussed in ferropnictides.

Fe magnetic impurity effect in Au atomic sized conductor

We have studied the electrical conductance in Au nanowire containing 0.07 at.%Fe ions (AuFe nanowire) with mechanically controllable break junction technique to investigate the magnetic impurity effects in the atomic-sized conductance. At room temperature, we observe not only conductance steps at the integer multiples of G0 = 2e2/h, but also steps deviating from them in AuFe nanowire. Moreover, these features persist down to T = 4.2 K. The scattering between the conduction electrons and Fe magnetic ions may lift the spin degeneracy of the transmission probability, which is responsible for the deviation. Zero bias anomaly is observed in the AuFe nanowire with the contact diameter larger than ~3 nm in the current-voltage (I-V) measurements at T = 4.2 K, which may be caused by Kondo effect.

The local effect of magnetic impurities on superconductivity inCoxNbSe2 andMnxNbSe2 single crystals

We investigate the effect of individual atomic impurities on the superconductingstate that they are embedded in. Using low temperature scanning tunnelingmicroscopy and spectroscopy we could identify Co and Mn atoms in theCoxNbSe2 andMnxNbSe2 single crystals and observe the influence on the local electronic density of states (LDOS) at0.4 K. We find that Co is in the weak scattering limit. In this case the LDOS is quitehomogeneous on the sample surface, despite the number of defects, and retains sharpcoherent superconducting peaks. This is in strong contrast to the effects of Mn impurities,which locally destroy superconductivity. In this case the LDOS shows a strongenhancement of spectral weight inside the superconducting gap even far from the Mnatoms. Moreover, two impurity bound states are found within the superconducting gap atE/Δ0 = 0.18 and 0.36 at locations close to defects.

Non-magnetic and magnetic impurity effects on superconductivity in the ternary iron-silicide [formula omitted

We studied effect of non-magnetic and magnetic impurities on superconductivity in Lu2Fe3Si5 by investigating superconducting properties of (Lu1-xRx)2Fe3Si5 (R=Sc,Y, and Dy). The rapid depression of Tc by non-magnetic impurities reveals strong pair breaking by disorder, providing compelling evidence for the sign reversal of the superconducting order parameter in Lu2Fe3Si5.

The Effect of Magnetic Impurities on the Response of Superconducting Photon Detectors

In a superconductor with magnetic impurities, scattering of conduction electrons results in the formation of localized states inside the superconducting gap. We show that inelastic electronic transitions involving quasiparticle scattering into and out of the localized states result in significant changes in the non equilibrium properties of the superconductor. The individual processes are: quasiparticle trapping into discrete states, enhanced recombination with localized quasiparticles, and pair breaking and de-trapping of localized quasiparticles by sub-gap phonons. We find that all these processes give rise to clearly distinguishable temperature dependences of the kinetic parameters indicating a possible role of magnetic scattering on the response of a photon detector.

Magnetic impurities in the pnictide superconductor Ba1−xKxFe2As2

Nuclear magnetic resonance (NMR) measurements have been performed on single crystals of Ba1−xKxFe2As2 (x=0, 0.45) and CaFe2As2 grown from Sn flux. The Ba-based pnictide crystals contain significant amounts of Sn in their structure, ∼1%, giving rise to magnetic impurity effects evident in the NMR spectrum and in the magnetization. Our experiments show that the large impurity magnetization is broadly distributed on a microscopic scale, generating substantial magnetic field gradients. There is a concomitant 20% reduction in the transition temperature, which is most likely due to magnetic electron scattering. We suggest that the relative robustness of superconductivity (x=0.45) in the presence of severe magnetic inhomogeneity might be accounted for by strong spatial correlations between impurities, such as clustering on the coherence length scale.

Iron-Based Layered Compounds: The Effect of Negative Interband Coupling

The experimental data relative to new superconductor LaFeAsO1−xFx can be analyzed in the framework of the two-band Eliashberg theory. With reasonable assumptions on input parameters of the theory such as electron–phonons and electron-antiferromagnetic spin density fluctuations spectral functions and the Coulomb pseudopotential, it is possible to calculate mean physical quantities such as critical temperature, superconductive gaps, superconductive differential conductance, and so on. In this model also the effect of disorder and magnetic impurities on the critical temperature is examined. The key ingredient is the negative interband coupling that can remarkably increase the critical temperature. This effect stems from the presence of an electron–phonon coupling constant smaller than the sum of the Coulomb pseudopotential and electron-antiferromagnetic spin density fluctuations coupling constant in the interband channel. The resulting superconductive state is an example of extended s-wave pairing with a sign reversal of the order parameter between different Fermi surface sheets.

Impurity effect of an S = 1/2 two leg spin ladder antiferromagnet [Ph(NH3)]([18]crown-6)[Ni(dmit)2] studied by ESR

Organic magnet [Ph(NH3)](18-crown-6)[Ni(dmit)2] is a candidate for an S = 1/2 two leg spin ladder system [S. Nishihara, T. Akutagawa, T. Hasegawa, and T. Nakamura, Chem. Commun. (Cambridge) 2002, 408.]. In order to investigate the non magnetic impurity effect for this compound, we performed X-band ESR measurement of [Ph(NH3)](18-crown-6)[Ni(dmit)2]1-x[Au(dmit)2]x. Non magnetic impurity concentration were estimated to be x = 0.029. ESR measurements have been performed from T = 4.5 K to 273 K using the Bruker ESR spectrometer. At T = 4.5 K, the fine structure ESR signals were observed. These signals suggest the existence of ferromagnetically coupled S = 1/2 dimers in the system.

Effects of magnetic impurity scattering on superfluid 3He in aerogel

We investigate impurity effects on superfluid 3He in aerogel whose surface is not coated with 4He, different from most experimental situations. In systems with no 4He coating, spins of solid 3He absorbed on the aerogel surface are active and interact with spins of quasiparticles relevant to superfluidity and, for this reason, such an aerogel is treated as magnetic scatterers. It is found that, in the ABM pairing state affected by magnetic scatterings, not only the l-vector but also the d-vector has no long-ranged orientational order, and that the strong-coupling correction due to impurity scatterings is less suppressed than that in the nonmagnetic case, implying an expansion of the A-like phase region.

Organic spin valves: effect of magnetic impurities on the spin transport properties of polymer spacers

We report the effect of magnetic impurities in the spacer layer of polymeric spin valves (PSV) with the sandwich configuration of La0.67Sr0.33MnO3(LSMO)/π-conjugated polymer regio-random poly(3-hexyl thiophene)/cobalt (Co), showing giant magnetoresistance (GMR) response. Different deposition rates of Co at the top electrode resulted in two types of devices: one with lower device resistance and linear current–voltage (I–V) characteristics and the other with very low inclusion of Co and exhibiting higher device resistance and nonlinear I–V characteristics. We observed an asymmetric dc bias dependence of magnetoresistance (MR) in devices with more Co inclusion, while for the other type of device, bias dependence was more symmetric. At higher bias, %MR of both types of device showed no significant difference (5–10%), but at low dc bias it ranged between 50 and 160% MR. This can be attributed to the higher tunneling probability of spin-polarized carriers from one ferromagnetic electrode to the other. Magnetic tunnel junction-like features are observed in the devices with greater Co inclusions. Anomalous MR peaks were also observed in these devices and their origin was explained in terms of presence of additional scattering centers around the included metal ions and increased spin relaxation due to high magnetic anisotropy in the system. Both types of PSVs showed a monotonic decrease in MR with temperature at high bias currents.

Spin- and charge-density-wave orders in La 1.87Sr 0.13Cu 0.99Fe 0.01O 4

We have performed elastic neutron-scattering measurements on La 1.87Sr 0.13Cu 0.99Fe 0.01O 4 to study the magnetic impurity effects on the stripe correlations in high- T c cuprates. Both charge-density-wave (CDW) and spin-density-wave (SDW) orders are observed in the low-temperature-orthorhombic (LTO) phase for the first time. With decreasing the temperature, CDW order first appears at ∼ 60 K followed by the SDW order at lower temperature of ∼ 50 K . The incommensurability for the CDW order ( ε ) was found to be 0.224 ± 0.002 r.l.u., which is approximately twice of that for SDW order ( δ ) of 0.115 ± 0.003 r.l.u. Both ordering sequence and relation of ε ≈ 2 δ are the same for SDW and CDW orders observed in the low-temperature-tetragonal (LTT) phase of La 1.60 - x Nd 0.4 Sr x CuO 4 (LNSCO), suggesting that the similar stripe correlations exist in the cuprate oxides, irrespective of crystal structure.

Donald Maurice Ginsberg

Donald Maurice Ginsberg, a world authority on superconductivity, passed away peacefully on 7 May 2007 at his residence in Urbana, Illinois.Ginsberg was born 19 November 1933 in Chicago. At the University of Chicago, he received both a BA in liberal arts in 1952 and a BS in physics in 1955. He joined the department of physics at the University of Illinois at Urbana-Champaign in 1959, immediately after finishing his graduate work with Michael Tinkham at the University of California, Berkeley; the title of his thesis was “Far Infrared Transmission Through Superconducting Films.”Ginsberg’s research was in superconductivity, and beginning in the late 1950s, he played a major role in almost every fundamental aspect of the field. Building on his far-IR graduate experience, he helped develop planar quasiparticle tunneling spectroscopy as a vital probe of the energy gap of metallic superconductors. Through a wide variety of measurements, including electronic transport, thermal conductivity, specific heat, and magnetic susceptibility, he contributed greatly in the 1970s to the understanding of the effects of magnetic and nonmagnetic impurities on the electronic structure of superconductors. In the 1980s he focused on the molybdenum chalcogenides, perhaps the most complex superconducting materials known at the time.Following the discovery of cuprate superconductors in 1986, Ginsberg was quick to reorient his lab in that direction. After trying every published recipe for creating good samples, he was dissatisfied, so he finally developed his own approach. He claimed that the most important hint for the best recipe came from a chance airport encounter with a colleague. His samples of yttrium barium copper oxide were universally acknowledged at the time to be the world’s finest. Because of the sensitivity of d-wave superconductivity to impurities, the advance in sample quality was essential for a series of fundamental experiments that established several novel effects, including fluctuation effects. With a group led by one of us (Van Harlingen), Ginsberg established the d-wave symmetry of the superconducting state. But not only did he make careful transport and equilibrium thermodynamic measurements, he generously shared his unique crystals with colleagues at numerous institutions around the world despite the intense competition in the field.For the past four decades, Ginsberg authored the Encyclopaedia Britannica’s article on superconductivity. He also wrote several influential review articles and book chapters, starting with his review, with L. Charles Hebel, on non-equilibrium properties of superconductors, which was published in the seminal two-volume set Superconductivity (Marcel Dekker, 1969). Beginning in the late 1980s, he edited the definitive five-volume series Physical Properties of High Temperature Superconductors (World Scientific), to which he also contributed. In addition, he published more than 240 papers with many hundreds of coauthors from two dozen domestic and foreign institutions.For his work on high-temperature superconductivity, Ginsberg received the 1998 Oliver E. Buckley Condensed Matter Prize of the American Physical Society, which he shared with Van Harlingen and with John Kirtley and Chang Tsuei of the IBM Thomas J. Watson Research Center. Among his other honors were a Sloan Foundation Research Fellowship and Illinois’s Tau Beta Pi Daniel C. Drucker Eminent Faculty Award and University Scholars Program award. At Illinois, Ginsberg was widely recognized for outstanding classroom teaching. Thirty-six PhD students did their research under his direction. A resourceful and clever physicist with a uniquely multifaceted personality, he was equally appreciated for his science and for his wide range of interests and his direct and often humorous way of expressing his thoughts.Ginsberg enjoyed the cultural side of life—especially poetry, which he wrote, and music, which he indulged by playing the flute. After his retirement in 1996, he wrote several books of poetry featuring his whimsical observations of physics, physicists, and his personal life. No event was too small to be lampooned by his fiendishly dry wit and droll turn of phrase. In considering his own long career at Illinois, Ginsberg remarked simply to a colleague, “When they ask about the old days, just tell them we had a good time.”Donald Maurice GinsbergPPT|High resolution© 2008 American Institute of Physics.

Magnetic impurity effects on quantum teleportation via the three-qubit Heisenberg XX model

We study the quantum teleportation through the three-qubit Heisenberg XX model with a magnetic impurity. The main result is the systematic analysis of the average fidelity's evolution as a function of magnetic impurity B and temperature T. What deserves mentioning here is that the effects of magnetic impurity on the average fidelity obviously behave differently whether the impurity is located on the receiver's site or not.

Chemical and magnetic impurity effects on electronic properties of semiconductor quantum wires

We present a theoretical study of electronic states in magnetic and nonmagnetic semiconductor quantum wires. The effects of chemical and magnetic disorder at paramagnetic temperatures are investigated in single-site coherent potential approximation. It is shown that the nonmagnetic impurity shifts the band of carriers and suppresses the van Hove singularities of the local density of states (LDOS) depending on the value of impurity concentration. The magnetic impurity, however, broadens the band which depends on the strength of exchange coupling, and in the high impurity concentration, the van Hove singularities in the LDOS can completely disappear and the curves become smooth.

Effects of Magnetic Impurities on Two-Band Superconductor

The calculation was made of the effect of magnetic impurities on the normal and superconductive properties of a multiband s-wave superconductor by direct solution of the two-band Eliashberg equations. In this way, the critical temperature was determined, as well as the values of the superconductive gaps, and the shape of the superconductive density of states and other physical quantities that depend on the concentration of magnetic impurities. It was found that the gaps and the penetration lengths display an unusual behaviour as a function of temperature. The possibility is examined that the presence of a negative induced gap raises the critical temperature.

Magnetic impurities in a superconductor: Effect of domain walls and interference

We consider the effect of magnetic impurities, modeled by classical spins, in a conventional superconductor. We study their effect on the quasiparticles, specifically on the spin density and local density of states. As previously emphasized, the impurities induce multiple scatterings of the quasiparticle wave functions, leading to complex interference phenomena. Also, the impurities induce quantum phase transitions in the many-body system. Previous authors studied the effect of either a small number of impurities (from 1 to 3) or a finite concentration of impurities, typically in a disordered distribution. In this work, we assume a regular set of spins distributed inside the superconductor in such a way that the spins are oriented, forming different types of domain walls, assumed stable. This situation may be particularly interesting in the context of spin transfer due to polarized currents traversing the material.

Effect of doping on the pseudogap enhancement due to magnetic impurities in d‐density waves

AbstractWe study the effect of quantum magnetic impurities on d‐wave spin density waves (d‐SDW). The impurity spins are aligned coherently according to the spin space anisotropy of the condensate. Both the order parameter and transition temperature increases due to the coherent interplay between magnetic scatterers and d‐SDW at fixed doping. By increasing the doping, the d‐SDW collapses. This can explain the recent experimental data on the pseudogap enhancement of Ni substituted NdBa2{Cu1−y Niy }O6.8 from Pimenov et al. [Phys. Rev. Lett. 94, 227003 (2005)]. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Pseudogap enhancement due to magnetic impurities ind-density waves

We study the effect of quantum magnetic impurities on d-wave spin density waves (d-SDW). The impurity spins are aligned coherently according to the spin space anisotropy of the condensate. Both the order parameter and transition temperature increases due to the coherent interplay between magnetic scatterers and d-SDW. This can explain the recent experimental data on the pseudogap enhancement of Ni substituted NdBa_2{Cu_{1-y}Ni_y}O_6.8 from Pimenov et al. (Phys. Rev. Lett. 94, 227003 (2005)).

Magnetic excitation and Na-deficient effects on spin-gapped compound [formula omitted

We studied magnetic excitations of a spin-gapped compound Na 2 Cu 2 TeO 6 , the hole doping effect on the Cu 2 TeO 6 plane via topotactic Na deficiency introduced by soft-chemical treatment, the spin vacancy effect by Zn substitution and the magnetic impurity effect by Ni substitution for Cu by means of magnetic susceptibility measurements. It turned out that unpaired moments of the doped holes by Na deficiency, Zn and Ni impurities induce Curie magnetism and lower the broad maximum temperature T max in the uniform spin susceptibility. The decrease of T max indicates the decrease of a predominant superexchange interaction.

Recent achievements in MgB2 physics and applications: A large-area SQUID magnetometer and point-contact spectroscopy measurements

In the first part of the present paper we discuss the fabrication and the characterization of an MgB2-based SQUID magnetometer with a directly coupled large-area pick-up loop, made on an MgB2 film deposited by an all in situ technique. The coarse structure of the SQUID was defined by optical lithography and Ar-ion milling, while the two nanobridges acting as weak links in the superconducting loop were made by focused ion beam (FIB) milling. The device was characterized at different temperatures and showed Josephson quantum interference up to 20K as well as a noise level already compatible with the recording of an adult magnetocardiogram.In the second part, concerning the fundamental physics of MgB2, we present the results of very recent point-contact measurements on Mg1−xMnxB2 single crystals with 34.1⩾Tc⩾13.3K (i.e. 0.37%⩽x⩽1.5%). The experimental conductance curves were fitted with the generalized two-band BTK model and their behaviour in magnetic fields was studied to check if both the order parameters (OPs) of the σ and π bands were present in the whole doping range. The dependence of the OPs (evaluated through the fit) on the Andreev critical temperature of the junctions is analyzed in the framework of the two-band Eliashberg theory by including the effects of magnetic impurities. The results give an evidence of a dominant effect of the magnetic impurities on the σ-band channel.